Cannot upgrade FROM High Serra 10.13.6 to Mojave 10.14.6?
If you have internet, you could boot to recovery and install the latest OS that your Mac is able to run (which is 10.14.6 right now). Power off your Mac and hold the following keys only right after you power on and until you see the Apple or the globe. You will need to connect to the internet (wired or wireless) for this to work.


Option-⌘-R


https://support.apple.com/en-us/HT204904

That will get you a free and clear installer that will run through the checks and if it fails, you will also have the ability to run Disk Utility from the installer to repair things if this is just a filesystem error causing the grief.

The Self Service app means you might be on an MDM managed application like with JAMF Pro, so if this is a work machine, be sure to check with your team to be sure there isn't a restriction preventing the upgrade.