The AG are using "Database Mirroring" Endpoint to communicate between all nodes.
Your SQL Service account need to have "connect" permission on the endpoint.
You also need to grant "ConnectSQL" to the other node service account (or the Server - AITS\PLSBDATEST2$- if using local service account).
So to recap, on PLSBDATEST1, you will have a login for ...
The account that is listed in the error message is the Active Directory computer account. If possible, your best solution is to reinstall SQL Server on all nodes and use the same AD account for the SQL Server service.
If you can't do that, create a login on the secondary(s) for AITS\PLSBDATEST2$ and add it to the sysadmin role.
With all high-availability solutions you will always have some down-time during a failover, typically less than a minute.
The reason is that the clusterware has to determine reliably that a member is down before it can fail over. That means that failover is only initiated after several attempt to communicate with the cluster member have failed.
You can ...