New client install on a DB server.
Installed latest version of pmm2_client using yum.

"pmm-admin config" appears to work : no errors reported.

"pmm-admin add mysql" fails with the error:
"Failed to get PMM Server parameters from local pmm-agent: pmm-agent is not connected to PMM Server."

Running the 'config' and 'add' with debug gives the following:

[root@[hostname] ~]# pmm-admin config --debug --force --server-insecure-tls --server-url=https://admin:'[password]'@[pmmServerIP]:443 [agentServerIP] generic [nodeName]
DEBUG 2021-08-18 13:04:34.17419395Z: Running: pmm-agent --server-address=[pmmServerIP]:443 --server-username=admin --server-password=[password] --server-insecure-tls --debug setup --force --metrics-mode=auto [agentServerIP] generic [nodeName]
DEBUG 2021-08-18 13:04:36.127131469Z: Result: &commands.configResult{Warning:"", Output:"Checking local pmm-agent status...\npmm-agent is running.\nRegistering pmm-agent on PMM Server...\nRegistered.\nConfiguration file /usr/local/percona/pmm2/config/pmm-agent.yaml updated.\nReloading pmm-agent configuration...\nConfiguration reloaded.\nChecking local pmm-agent status...\npmm-agent is running."}
DEBUG 2021-08-18 13:04:36.127176577Z: Error: <nil>
Checking local pmm-agent status...
pmm-agent is running.
Registering pmm-agent on PMM Server...
Registered.
Configuration file /usr/local/percona/pmm2/config/pmm-agent.yaml updated.
Reloading pmm-agent configuration...
Configuration reloaded.
Checking local pmm-agent status...
pmm-agent is running.
[root@[hostname] ~]# pmm-admin add mysql --debug --username=[pmm2_user] --password=[password] --query-source=perfschema --service-name=[nodeName] --host=127.0.0.1 --port=3306 --environment=PROD
DEBUG 2021-08-18 13:04:56.745545862Z: POST /local/Status HTTP/1.1
Host: 127.0.0.1:7777
User-Agent: Go-http-client/1.1
Content-Length: 3
Accept: application/json
Content-Type: application/json
Accept-Encoding: gzip

{}

DEBUG 2021-08-18 13:04:56.747133428Z: HTTP/1.1 200 OK
Content-Length: 412
Content-Type: application/json
Date: Wed, 18 Aug 2021 13:04:56 GMT
Grpc-Metadata-Content-Type: application/grpc

{
"agent_id": "/agent_id/dcded051-d223-4068-b821-1421d9809f0e",
"runs_on_node_id": "",
"server_info": {
"url": "https://admin:[password]@[pmmServerIP]:443/",
"insecure_tls": true,
"connected": false,
"version": "",
"latency": null,
"clock_drift": null
},
"agents_info": [
],
"config_filepath": "/usr/local/percona/pmm2/config/pmm-agent.yaml",
"agent_version": "2.20.0"
}
Failed to get PMM Server parameters from local pmm-agent: pmm-agent is not connected to PMM Server.
[root@[hostname] ~]# pmm-agent --debug
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/node_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/mysqld_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/mongodb_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/postgres_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/proxysql_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/rds_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/azure_exporter component=main
INFO[2021-08-18T15:05:05.716+02:00] Using /usr/local/percona/pmm2/exporters/vmagent component=main
DEBU[2021-08-18T15:05:05.716+02:00] Loaded configuration: &{ID: ListenAddress:127.0.0.1 ListenPort:7777 Server:{Address: Username: Password: InsecureTLS:false WithoutTLS:false} Paths:{ExportersBase:/usr/local/percona/pmm2/exporters NodeExporter:/usr/local/percona/pmm2/exporters/node_exporter MySQLdExporter:/usr/local/percona/pmm2/exporters/mysqld_exporter MongoDBExporter:/usr/local/percona/pmm2/exporters/mongodb_exporter PostgresExporter:/usr/local/percona/pmm2/exporters/postgres_exporter ProxySQLExporter:/usr/local/percona/pmm2/exporters/proxysql_exporter RDSExporter:/usr/local/percona/pmm2/exporters/rds_exporter AzureExporter:/usr/local/percona/pmm2/exporters/azure_exporter VMAgent:/usr/local/percona/pmm2/exporters/vmagent TempDir:/tmp PTSummary:/usr/local/percona/pmm2/tools/pt-summary PTPgSummary:/usr/local/percona/pmm2/tools/pt-pg-summary PTMySqlSummary:/usr/local/percona/pmm2/tools/pt-mysql-summary PTMongoDBSummary:/usr/local/percona/pmm2/tools/pt-mongodb-summary SlowLogFilePrefix:} Ports:{Min:42000 Max:51999} Debug:true Trace:false Setup:{NodeType: NodeName: MachineID: Distro: ContainerID: ContainerName: NodeModel: Region: Az: Address: MetricsMode: DisableCollectors: CustomLabels: Force:false SkipRegistration:false}} component=main
INFO[2021-08-18T15:05:05.716+02:00] Starting... component=client
DEBU[2021-08-18T15:05:05.717+02:00] Starting gRPC server on http://127.0.0.1:45317/ ... component=local-server/gRPC
DEBU[2021-08-18T15:05:05.717+02:00] Reflection and channelz are enabled. component=local-server/gRPC
ERRO[2021-08-18T15:05:05.717+02:00] PMM Server address is not provided, halting. component=client
INFO[2021-08-18T15:05:05.717+02:00] Starting local API server on http://127.0.0.1:7777/ ... component=local-server/JSON
DEBU[2021-08-18T15:05:05.717+02:00] Debug handlers:
http://127.0.0.1:7777/debug/metrics
http://127.0.0.1:7777/debug/vars
http://127.0.0.1:7777/debug/requests
http://127.0.0.1:7777/debug/events
http://127.0.0.1:7777/debug/pprof component=local-server/JSON
INFO[2021-08-18T15:05:05.719+02:00] Started. component=local-server/JSON
PANI[2021-08-18T15:05:05.719+02:00] listen tcp 127.0.0.1:7777: bind: address already in use component=local-server/JSON
panic: (*logrus.Entry) 0xc0003a3e30

goroutine 133 [running]:
github.com/sirupsen/logrus.Entry.log(0xc0001a2cb0, 0xc000130d80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
/tmp/go/pkg/mod/github.com/sirupsen/logrus@v1.4.2/entry.go:239 +0x2ea
github.com/sirupsen/logrus.(*Entry).Log(0xc000282cb0, 0x0, 0xc000341f90, 0x1, 0x1)
/tmp/go/pkg/mod/github.com/sirupsen/logrus@v1.4.2/entry.go:268 +0xf0
github.com/sirupsen/logrus.(*Entry).Panic(0xc000282cb0, 0xc000341f90, 0x1, 0x1)
/tmp/go/pkg/mod/github.com/sirupsen/logrus@v1.4.2/entry.go:306 +0x55
github.com/percona/pmm-agent/agentlocal.(*Server).runJSONServer.func2(0xc000282cb0, 0xc0000260e0)
/tmp/go/src/github.com/percona/pmm-agent/agentlocal/agent_local.go:297 +0xf2
created by github.com/percona/pmm-agent/agentlocal.(*Server).runJSONServer
/tmp/go/src/github.com/percona/pmm-agent/agentlocal/agent_local.go:294 +0xcf3

Noting the error :  listen tcp 127.0.0.1:7777: bind: address already in use component=local-server/JSON 

I checked the port usage:

[root@[hostname] ~]# netstat -nltp | grep 7777
tcp 0 0 127.0.0.1:7777 0.0.0.0:* LISTEN 29361/pmm-agent