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:
4. Go to your site and add the KWizCom Modern Experience On Prem
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:
5. After the KWizcom Modern Experience On Prem has been added to your site, you can now activate the KWizCom modern apps in the relevant sites. Please refer to "installing KWizCom modern apps in your SharePoint 2019 site"