You are trying to install ShortPoint on SharePoint 2019 server and got below error while Adding ShortPoint App to your site:

Sorry, apps are turned off. If you know who runs the server, tell them to enable apps.

Reasons and Solutions

Apps are not enabled in your farmMake sure your have Apps enabled in your SharePoint 2019 farm.
Subscription service is not setupThe first and most probably problem would be the subscription service is not configured in your farm. Please follow our article's PART 1: Configure Subscription Settings Service Application to configure it.
App management service is not associated with your web applicationMake sure App Management Service Application is associated with your web application where you want to add ShortPoint App.
App Url is not configuredChances are you have not configured the App Urls. Please ensure you have given values to App Domain and App Prefix in your App Store settings.

If after ensuring above solutions, you still have the same error, please register a ticket with ShortPoint in order to let our technicians analyze the problem.