
We cannot convert accounts that are not active in AD from classic to claims.
If the user is disabled or deleted in Active Directory, check as following to delete the user in User Information List.
Please navigate to the site collection>edit the URL by adding the following string to the end of it:
/_layouts/15/people.aspx?MembershipGroupId=0
Select the person from the list, and then on the Actions menu, select Delete Users from Site Collection:
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