Hi Alice,
May I know what type of Microsoft Teams are you using? Is it Teams free?
Regarding your problem, we have received some feedback about the problem "The specified list is Invalid. The specified list might have been deleted or you might not have permission to view it. Contact the site owner for more information". However, we diagnosed and found that the issue is with user's ISP. When users switched to a different network, they were able to access the files from Teams App as well as the SharePoint Admin portal successfully. You may first try to change to another network to see if the problem can be resolved. And please also contact your admin and let them check if there any block in your firewall configuration which may also cause such a problem.
Best Regards,
Shawn