Solid but will not subtract

I’m trying to subtract from landscape, I’ve done it before but now it will not subtract. It gives me short edges error and I can fix it by changing the threshold, but sketchup wont recognize it as solid. Help

Impossible to tell what the issue is from your screenshot. Share the .skp file.

People have been getting hung up on short edges. A solid can have short edges and still be a solid, Thomthom added the short edge detection as an added bonus tool that ‘can find short edges for you’ but they are only relevant if they are causing issues in your geometry, like too small and faces are not forming. If your geometry is pretty straightforward some short edges are meaningless, and especially so if you have the threshold set too high. Short edges real only start to become an issue when less than 1mm.
Generally speaking you can turn off short edge detection and all will be fine. Turn it on again if you really need to troubleshoot a non solid small object or very complex mesh.
GIF 15-04-2024 9-41-58 AM

And as @DaveR already said, we need the model to troubshoot further.