In order to make Apps available to be added to your SharePoint 2019 farm, you need to enable Apps in your SharePoint farm. Here are the steps to do so:

Step 1: Go to Central Administration 

Open SharePoint 2019 Central Administration

From your SharePoint Server 2019 farm, open SharePoint 2019 Central Administration

Step 2: Go to Store Settings

Navigate to Apps and Configure Store Settings

Step 3: Make sure the App Purchases are enabled

Make sure you have Yes for Should end users be able to get apps from the SharePoint Store?

Thank you!