Mac Mini/Mojave Install

Your Mac mini has experienced a "kernel panic." This can be caused by a number of reasons and potentially prevented you from upgrading it to macOS Mojave. Which exact version is your mini and what version of OS X or macOS is it running before you attempted to upgrade it?

I suggest that you try to following:

1. Remove any peripherals, except the keyboard, mouse, and monitor, and then, try restarting your mini again.
2. Try booting up your mini in Safe Mode: Use safe mode to isolate issues with your Mac - Apple Support
3. Reset your mini's SMC:  How to reset the System Management Controller (SMC) on your Mac - Apple Support

Please post back your results.

That is a kernel panic which generally indicates a hardware problem.

You can ask them.

In my neighborhood there are three Apple Stores. I know one is alway so busy that they are not nearly as helpful as the other two.

Yes. They should be able to help you get this resolved.