1. Download and Install SharePoint Online Management Shell.
2. Login into you tenant site
Connect-SPOService https://<your-domain>-admin.sharepoint.com/3. Run the following command to enable App Catalog on a specific site
Add-SPOSiteCollectionAppCatalog -Site https://<your-domain>.sharepoint.com/sites/<your-site>4. App Catalog is enabled now, you can access it from site contents or from the following link
https://<your-domain>.sharepoint.com/sites/<your-site>/AppCatalog/Forms/AllItems.aspx