We put a 12' inflatable pool on a screened in lanai. Problem is the lanai slants and the pool won't fill to fill line. Any suggestions on how to level pool?

I think you'll have to shim up the pool. How much of it rests on the slanting lanai?

Level the floor beneath it with either sand or wood.

if that's not an on-ground lanai (patio) then you've overloaded it, and were lucky it didn't collapse. otherwise, ron45 has your answer. wood is likely to poke a hole in the liner, use sand.