An Equipment without reconfigure that's also a creature can't be attached to anything. You can activate its equip ability, but it won't become attached. For example, if you control both Red Herring and Armed with Proof, Red Herring will be an Artifact Creature – Clue Equipment Fish with "Equipped creature gets +2/+0" and equip {2}, but activating its equip ability won't result in it becoming attached to the target creature.