Re: ‘Invalid Object’ when trying to change walk height.
Just a guess without seeing what the build looks like, but it is probably the stairs. When you change the wall heights, the stairs will change to accommodate the new wall height. If there is an object in the tiles that stair reconfiguration needs so it can place properly, you will get that or a similar error message. Walls and objects around the stairs on both floors will cause it so you need to look on both floors involved. So I suggest, before you change the wall height, delete the stairs and once the room height is changed to what you want it to be, then put the stairs back. You might have to rearrange walls and objects to get them back in. In the case of the pool, you might have to delete and replace that too.
It any of that doesn't work, if you post a few screenshots of what the build looks like, it would be helpful to help you further. And if you are willing to upload the build to the Gallery, I or another helper can take a look at it to see if we can find what is causing the issue. Just let us know here what the lot name is so we can find it on the Gallery.
Hope this helps.