Hi @IniobongNkanga-8038,
Your files have been checked out already, so it's an expected behavior when you try to check out the file. And if you log in with other users, you will be unable to check in the files unless you are the owner of the site. You could get who checked out the files by following steps
https://sharepointmaven.com/how-to-check-in-documents-on-behalf-of-others-in-sharepoint-online/
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