
Hi @IBN
Welcome to Q&A forum!
I also have the same idea with you. File permissions in Teams private channel may block this user to upload file to Teams.
Before troubleshooting this problem, please check this user is member or owner in channel. If the user is member, it recommends you try to promote this user to owner to see if it can be fixed.
Meanwhile, you may try to check the permissions of the affected channel through SharePoint.
You can follow the following steps:
a) As a owner of the private channel, click on the channel > Files > Open in SharePoint. The channel SharePoint window will show up.
b) Click on Settings > Site permission > Advanced permissions settings.
c) Check if the Permission Levels of private channel members is Contribute. If not, click Edit Permissions to assign the Contribution to them > OK.
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