Domain joined failed

I assume you have gone through the steps listed in https://docs.aws.amazon.com/appstream2/latest/developerguide/troubleshooting-notification-codes.html for DOMAIN_JOIN_INTERNAL_SERVICE_ERROR

If not for the usual suspects as documented previously, this is likely due a Microsoft Security update. Please follow the guidance in the link here https://support.microsoft.com/en-gb/topic/kb5020276-netjoin-domain-join-hardening-changes-2b65a0f3-1f4c-42ef-ac0f-1caaf421baf8

If you are using Managed AD and cannot implement the suggestions in the link, i would suggest the following:

1. Power down the fleet
2. Delete all the Computer Objects in the AppStream OU.
3. Power On the fleet and let the configured AppStram Service Account create objects. What this does is to satisfy the condition in the article "The user attempting the operation is the creator of the existing account" which will help mitigate this issue.
4. Dont change the Service Account.

If the above doesnt work, please contact AWS Support