Regarding the "App Catalog" in SharePoint 2019

Please go through the below checklist to troubleshoot and ensure everything is set up correctly:

In SharePoint Online, there’s an option to deploy an app to all sites. In SharePoint 2019, you may need to add the app directly to the site where you want the customizations to appear, especially if tenant-wide deployment isn't available.

Please ensure the App Catalog site is fully functional and accessible without permissions issues.

Navigate to Site Contents in the site where you want to use the extension (http://win-xxxxx:19065/). Click on Add an App. Look for the app (sample-extension-client-side-solution) you deployed in the App Catalog. Select it and add it to the site.

In some environments, custom scripts might be restricted. Ensure that custom scripting is allowed on the site where you’re adding the app. Enable custom scripts in the SharePoint Admin Center or via PowerShell: 
Set-SPOsite -Identity "http://win-xxxxx:19065" -DenyAddAndCustomizePages 0
(Powershell script)

After adding the app, verify that the extension is activated on the list where you want it to appear

Go to the list where you expect the custom command set to show up, and check the command bar or list actions. It should appear automatically if everything is set up correctly.

Try clearing your browser cache or opening the site in a private browsing window.

If the customization still doesn’t appear, check the SharePoint ULS (Unified Logging Service) logs for any errors or warnings related to the app or the extension

Create a new list or site collection and try adding the app to see if the command set appears there. This helps to identify if the issue is site-specific.

Ensure the SPFx solution(Version) is compatible with SharePoint 2019