It is environment variable for a connectors instance or pod. The error you pasted indicates that your Connectors cannot connect to the Operate instance. This is required if you would like to support inbound capabilities, like webhooks.
Thus, you have two options - (1) disabling connectivity to Operate. In that case you have to pass OPERATE_CLIENT_ENABLED=false environment variable - more here, or
(2) enable Operate connectivity with OPERATE_CLIENT_ENABLED=true but you need also specify Operate URL and credentials depending on your setup.
Also, take a look at k8s deployment implementation for different modes - when Operate connectivity is disabled, using credentials without keykloak, or with keycloak (oauth).
Hope this helps!