I am not able to download Self Service

More details would be helpful. However, I believe I can make the following assumptions based on your screen shot and your limited question. I will assume you are using Jamf Pro since you are looking for "Self Service." By the way, love seeing the TestNav app. On day 9 of testing.

If you are using Jamf Pro, please be mindful that you must go to Apple School Manager and volume license the Self Service app. Make sure you license in excess of what you need. Remember volume licensing math... $0 * 10 is the same as $0 * 999. Always license free apps in excess of what you need. This way you don't need to come back to the well when you add more devices.

Once you confirm that you've volume licensed the app, go to Jamf Devices > Mobile Device Apps. Find Jamf Self Service in the list of apps. Make sure the app is set to Intall Automatically (other options are environment specific so decide on what you want set). But, most importantly, go to the App Configuration tab and drop in the app configuration. See this page from Jamf for details on how to configure and what to enter. Step 27 has the preference snippet you need. Simply copy and paste.

You may want to unscope the app, wait a few seconds, and then scope it to your devices again. Looks like your Jumpstart tech did not do a good job on the setup. This is usually covered during the Jumpstart.

Hope this is helpful. Please provide more details (without revealing personal, institutional, or environment specific) when asking for assistance.