As of the 17th of May, 2023, I discovered that the acceptance of user access from the Google Merchant Center immediately displays a 403 error.
This issue has persisted for a week, but there is no cause for alarm. Wait approximately 15 minutes before attempting to reopen the Google Merchant Center. At this point, you can view the account.
When this problem presented itself, it caused a fair amount of concern as I had just purged one of my emails due to the restriction limiting a single email to 100 Google Merchant Center accounts. At that time, I was under the impression that this action might have been the cause of the 403 errors. However, upon further investigation, it appears that this issue is universal.
In the interim, I strongly recommend you report this issue to Google for their attention and remediation.