I created a gnome wizard and chose school of illusion at second level.

Gnome grants you minor illusion.

And school of illusion grants you minor illusion (or another cantrip if you already know it).

However, when I go to manage spells it won’t let me learn another cantrip. Does anyone know a fix to this?

You could create a private homebrew Feat that grants you a single Wizard cantrip of your choice.  Use the Magic Initiate (Wizard) feat as a template and just remove the extra spell options from it.

Works for me, thanks.