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.

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

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.