Magebane Armor can equip a creature that doesn’t have flying. The “lose flying” effect just won’t do anything in that case.

Magebane Armor causes the equipped creature to lose flying at the time it became equipped. Any ability that grants that creature flying that starts to work later (such as a Jump that resolves later, or a Levitation that comes under your control later) will work normally.

Combat damage is the damage that’s dealt automatically by attacking and blocking creatures, even if it’s redirected (by Harm’s Way, perhaps). Noncombat damage is any other damage (generally, it’s damage dealt as the result of a spell or ability).