If you choose a target creature and it’s an illegal target by the time Scale the Heights tries to resolve, the spell doesn’t resolve. You don’t gain life, draw a card, or get to play an additional land.

You don’t play a land as Scale the Heights resolves; Scale the Heights fully resolves first and you draw a card, perhaps including a land you’ll play later. If it’s not your turn, you won’t be able to play a land this turn at all.

The permission to play lands is cumulative with other effects that allow you to play additional lands, such as that of Nahiri’s Lithoforming.