Hi @totally_gizmo Per my test, the Master Page Gallery is restricted. When I try to upload a file I will get "Access Denied" error even if I am a Site Collection Admin. There is a "Deny" rule that restricts access to Master Page Gallery. And this rule is not a part of any Permission levels available on the site collection. We can set it using SharePoint Online Management Shell
Connect-SPOService https://&ltyourtenant>-admin.sharepoint.com Set-SPOSite -Identity https://<yourtenant>.sharepoint.com/sites/<team-site> -DenyAddAndCustomizePages $false
After that the permissions in Master Page gallery work as expected and the Deny rule is gone.
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