If the error is happening when adding a User
This issue can happen if you are adding a new user that already exists with a duplicate username. Sort the Users list by username and check if the user already exists. If they exist but are Inactive, you can simply re-activate the existing account.
If the error is happening when trying to access a CRM record:
Try accessing the record using an account with admin privileges.
If the admin user also cannot access the record, then the record has most likely been deleted from CRM.
If the admin user can access the record, then ensure the user has the correct permissions, by following the below steps.
Note: A system admin is required to perform the following steps:
2.1. Go to the Admin menu
2.2. Click User Management and search for the user.
2.3. Adjust the permissions to match other users that can access the record.
2.4. Click Save