If you add a SharePoint Online (SPO) Add-in or SharePoint Framework (SPFx) solution in a site collection catalog and you fin the error “Cannot enable the site collection app catalogue feature…” (+ details in the screenshot), in this blog post you will find a possible solution for that error:
The solution implies to do the following steps:
Execute Remove-SPOSiteCollectionCatalog cmdlet in the site collection where you have found that error (Note: This step might not be necessary).
Verify the user that created the App Site Collection catalog using the –SPOSiteCollectionAppCatalog cmdlet is a Site Collection Admin in the Global App catalog in the tenant.
Run again the Add-SPOSiteCollectionAppCatalog cmdlet for the Site Collection.
Try to upload again a SPFx solution to the Site Collection App Catalog and check this time it’s uploaded without problems.