Have you ever came across a scenario where OOB web parts are not visible to the “SubSite” site owners in SharePoint? I have seen this error various times over the years but never documented.
Recently one of my clients came across the same error, and I figure it’s worth to document both solution and reason behind it.
This is typical if you have subsites with permissions not inherited and site owners in subsite don’t have access to the root web in the site collection. Web part gallery is stored at the hidden document gallery in root web (e.g. https://sitecollection/_catalogs/wp) and if users don’t have access to this library, they won’t be able to see web parts.
The fix here is – configure “Read” permissions for all site owners on the root web, if you have secured subsites.