Hi there,
It is possible that you didn't correctly allowed the local computer account on which you are trying to install the AD group managed service account (gMSA) to obtain its plaint-text password. It means you didn't specify the machine account in the list of -PrincipalsAllowedToRetrieveManagedPassword parameter values of the New-ADServiceAccount or the Set-ADServiceAccount.
Without being able to retrieve the password, the servers which need to log on the group managed service account would not be able know its password and log the account on.
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