I've noticed that since I upgraded my MacBook to Mojave, my Excel ribbon's dropdown boxes are not dropping. For example, if I wish to change the background color of a cell, I can no longer click the fill command and get the dropdown box containing the color palette. Any suggestions as to what's wrong?

Start by repairing the installation (I assume that's an option in Mac). I've had similar issues with upgrades on Windows machines.

Fixed...there was an update that I hadn't installed yet. I guess the prior version wasn't 100% compatible with Mojave