Error: The credentials you provided for the Sql server agent service are invalid.
Quote from the similar case : https://social.msdn.microsoft.com/Forums/sqlserver/en-US/43e025f0-538c-4416-8950-468443759329/adding-second-node-to-sql-server-cluster-fails-credential-for-sql-server-agent-is-invalid?forum=sqldisasterrecovery
The error clearly states that the service account for SQL Server Agent service is not invalid. Please make sure you have input correct account name and password and also ensure that the Activity Directory Service (ADS) is running properly so that the account you use can be validated by ADS.
BR,
Mia
If the reply helped, please “Accept Answer” and upvote it.--Mia