Use Admin Tools to delete 1 (one) user to determine if it's related to the workload behind deleting users and moving all their content (if there's a lot).
If it fails on that one user, skip that one, and attempt to delete a different, single user.
If it still fails, please attempt to use Admin Tools to move all a user's content first, and then please attempt to use Admin Tools again to delete this user with no content.
If it STILL fails, please attempt to use your native ESRI platform (ArcGIS Online or Enterprise) to delete the user from the previous step with no content.
NOTE: If it fails after Step 4, this will need to be handled by Esri support, as there would be something wrong with the platform in which Admin Tools makes use of to work. In short, if an operation fails naively in ArcGIS, it will likely fail with Admin Tools, too.