Unable to recovery from enrollment of existing account to control tower

Hi There Have you tried moving the account to the root OU and then enrolling it?

From https://docs.aws.amazon.com/controltower/latest/userguide/troubleshooting.html#enrollment-failed

In this case, you must take two recovery steps before you can proceed with enrolling your existing account. First, you must terminate the Account Factory provisioned product through the AWS Service Catalog console. Next, you must use the AWS Organizations console to manually move the account out of the OU and back to the root. After that is done, create the AWSControlTowerExecution role in the account, and then fill in the Enroll account form again.

If that does not enable the Enroll button, then try creating a new OU, moving the account into that OU, and registering that OU. That will start the enrollment process again.