There different ways crank arms are secured, but guessing the type you most likely have.

Knock the crank arm on hard with a hammer. You may use a block of wood or something, so you don't damage the crank arm. You can lie the bike down, and put another block of wood under the other side to support it. Don't get so carried away that you bend the crank arm. Then tighten the bolt hard with an allen key.

It is possible that aluminum crank arms can become worn on the inside, and not fit properly, or stay tight. Most people then replace it. If it was me, I would try filling the gap with epoxy resin, knocking it on tight, and tightening the bolt. I have not done this, but it would probably be effective for a cheap bike.