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!