Note: See the article The SharePoint App Catalog at Site Collection level (https://guitaca.com/Blogs/Blog?id=4f889bc1-5274-7c1e-4571-e4b1b0ba2b4f) to get information about configuration and use of the App Catalog at Site Collection level.

If the App Catalog at the Site Collection level is not good configured, users will be not allowed to install correctly SPFx components. Two error should be detected:

1 - The Deployed column will show always No for any kind of SPFx component.

2 - The buttons in the Apps section of the Files tab in the App Catalog are not visible:

If correctly configured, the buttons will show as follow:

This happens when the account that runs the PowerShell account to create the App Catalog (Add-SPOSiteCollectionAppCatalog) is NOT set as Site Collection Administrator on the Site Collection.

In the cmdlet specifications (https://docs.microsoft.com/en-us/powershell/module/sharepoint-online/add-spositecollectionappcatalog?view=sharepoint-ps), Microsoft indicates:

    To run this cmdlet user must have the following roles:
    SharePoint Admin role or Global Administrator role
    Site Collection Administrator of the tenant app catalog
    Site Collection Administrator of the site collection of the site specified in the -Site parameter

The three conditions must be met. If not, the shown error will occur.

To fix the error, use the Remove-SPOSiteCollectionAppCatalog cmdlet to remove the local App Catalog using the parameter -Site pointing to the Site Collection URL. Then add the account used to run the cmdlet to the Site Collection Administrators collection, and finally run again the Add-SPOSiteCollectionAppCatalog cmdlet to create the App Catalog.