Dear Alana,
Regarding "When trying to access any files in their Teams Group.", may I know whether the issue happens when you click on the Files tab in Teams and when you access the associated document library in SharePoint Online?
If it is convenient, upload some related screenshots for our reference.
Note: Remove any private information before uploading the screenshots.
From the information "Several employees in my company are experiencing this message", it seems that the issue doesn't happen with all users. Could you contact the site owner and let him go to the connected site in SharePoint Online to see whether the library is removed?
If you aren't the administrator, I suggest you contact him use the Website tab to embed the library in Teams to see whether you can see the library in Teams.
Welcome to share any updates at your convenience.
Thank you for your effort and time.
Sincerely
Cliff | Microsoft Community Moderator