Unable to sign out of Apple Account due to restrictions

Go to Settings > Screen Time > Content & Privacy Restrictions > Account Changes, and set that to Allow.

If your device is part of a Family Sharing plan, then the Organizer will need to make that change if they have prevented you from signing out of your account.

• In the Screen Shot you posted, tap Family and it will show the members in the Family plan.
• To see if there are any management profiles installed on your phone that would prevent you from signing out, go to Settings > General > VPN & Device Management. If there are any profiles listed there, then those can be used to put Restrictions on your device.

When you followed the link provided in the previous post to view the Account Changes restriction, was it currently set to Allow?