Your organization has configured the Site Collection App Catalog and you received following error while uploading the app to your site connection's Apps for SharePoint library:

Error: Cannot enable the site collection app catalog feature in this site collection because it has not been approved by the tenant admin

Reasons and Solutions

You did not wait for some time after converting your site collection to site collection app catalog    Although it looks instant from PowerShell however we many times faced this issue for no reason and the only resolution is to wait for a couple of hours after you convert your site collection to site collection app catalog.
Please check if your site collection is still in approved list of Site Collection App catalogThe tenant administrator might have removed the given site collection app catalog from approved list. In this case you can still see the Apps for SharePoint library but the apps to this library will not be functional. Please check with your tenant administrator and ask him / her to add the site back to approved list.

Tenant Administrator should also be site collection administratorThis is known bug from SharePoint. If the tenant admin who is approving any site collection as site collection app catalog is not the site collection administrator, you will receive above error. And the workaround is following:

1. Remove your site from site collection app catalog list
2. Add tenant admin (who is performing this step) to site collection administrators
3. Add the site back to approved list.