Hi,
It seems the task was done on one side that is OnPrem you also need to do this on the Entra side, check this article and see the process also backup the environment before you make changes as it seems you have uninstalled the AAD Connect and Sync is broken so better to backup servers and DCs so you dont have to rely on System restore.
https://www.alitajran.com/disable-active-directory-synchronization/
Hope this helps. JS
== Please Accept the answer if the information helped you. This will help us and others in the community as well.