After configuring the App Management service, you need to make sure the App URL for your apps is configured. Here are steps to ensure its configured as expected:
Step 1: Go to Central Administration
Open SharePoint 2019 Central Administration
From your SharePoint Server 2019 farm, open SharePoint 2019 Central Administration
Step 2: Navigate to Configure App URLs
Go to Apps and click on Configure App URLs
Make sure you have filled in App Domain and App Prefix here:
Once you provide these values, click OK as highlighted in above screenshot.
Note: While these settings are required to make sure you are able to use ShortPoint App, they are never used in ShortPoint SPFx Apps right now. Which means you need not to configure the domain in your DNS to provide here. You can give any value there right now.