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: Navigate to Service Application Associations

Go to Application Management -> Configure service application associations

Make sure the web application where you want to add ShortPoint App, has App Management Service associated to it:

If you don't see the association, please associate your App Management Service with you web application as follows.

Step 3: Associate App Management Service Application with your Service 

Click on Application proxy group available against your web application 

It will open a modal dialog box, make sure the App Management Service is Enabled

Press OK

Now you should see the App Management Service Application is associated to your web application where you wish to add ShortPoint App

Thank you!