IMPORTANT: Before you install any apps you must make sure your SP2019 server is properly configured to host apps, by following the MS documentation: https://docs.microsoft.com/en-us/sharepoint/administration/configure-an-environment-for-apps-for-sharepoint
Installation steps can be found below:
3. Go to your top-level site and add KWizCom Apps Foundation and add the product app to your site a. Go to the site where you want to have the app and go to “Site contents” b. Click “New” menu and select “App” c. Click “From Your Organization” link on the left side:
e. Add the product app to your site/s.