Hello and welcome to the forum
As one suggestion, did you check the integrity of the ISO you downloaded? Please see here.
An easier way of doing in this in Zorin is to download and install a program called "GtkHash," available from the repositories using your software manager.
It's also possible that UNetbootin did not load the files correctly on your USB drive. If UNetbootin is unable to work correctly on your system, you can also try using another tool. Please see the thread here
or this page.
Some of these might be available in the repositories.