I have an infinity pool built on the 5th floor of my house, but I cannot enter the pool.
I don’t have platforms built around it, and already installed a ladder to the pool, but I just can’t go inside the pool - a red cross shows up when I clicked the pool.
I have a pool built on the 2nd floor as well, but it works perfectly fine.
Does anyone know what causes this problem?
I’m currently using GFN on Mac to run the game.
Can your Zoi reach the white floor surface in front of the ladder? Or does the game cancel that movement too? Edit: Oops, I just noticed your Zoi standing on the white floor in your second screenshot. So at least that movement is OK.
It’s a little hard to see in the screenshots if there are differences in height between the brown floor, the white floor, and the ‘coping’ or edge which surrounds the pool. If there’s more than a small difference in height between levels, Zois won’t be able to move there.
A daytime shot showing those surfaces and their relative height, preferably with the sun angled to show shadows cast by the edges, would help determine if that’s actually the problem or it’s something else.
Hope this helps!
The brown floor is actually some outdoor floor panels. I have tried to remove the panels but still cannot use the pool. 
My Zoi can walk to the edge of the pool, just cannot interact with the pool, there is not even an error message like “you cannot move here”, just a red cross, so I don’t know what is the error leading to this…
This is the view in building mode without the panels, with the pool built on pure floor.
And this is the version with the panels.
Hmm, very strange. You can see how the brown floor panels sort of fill in the height difference between white floor and the edge of the pool, but you mention it doesn’t work either way. Hmm.
One other possibility comes to mind. You built an enclosed room on the 4th floor so you could later build the pool on top of it on the 5th floor, right? The game, as I’m sure you know, you’re obviously quite experienced with Build Mode, the game automatically generates floor and ceiling surfaces for an enclosed space. Did you lay out the pool on the ceiling surface of the room below, or did you add a floor surface on top of it? In other words, is what you’re thinking of as the floor, is it actually the ceiling surface?
If you just used the ceiling surface belonging to the room on the 4th floor, I wonder if that confused the pool tool. If so, perhaps adding a floor surface on top (temporary room just to auto-generate the floor) and then drawing out the pool on that would work better? Just making wild guesses here. 
You mentioned that the pool on the 2nd floor works fine. Can you determine any differences in how they are set up, or the sequence you used to create it? Maybe that would provide a clue.
I hope you can find a solution, the home you’re building looks pretty sweet!
I’ve tried re-building the pool on the 5th floor after reading your comment, but unfortunately, building the pool with or without the ceiling / floor surface seems not to be the reason to the problem
I tried to delete the functioning pool on the 2nd floor, and it seems building a pool will automatically “remove” the ceiling surface, so I guess the ceiling is not the problem 
I remembered when I built the house, I built an enclosed 5th floor first, drew out the pool area, before changing the walls around the pool into glass “fences”. There is now a floor surface which I can delete.
I even tried to remove the whole platform (as you can see from the stairs in the very first screenshot) I built on 5th floor to make the whole floor an even, flat surface, but still the pool is not functioning.
The weird thing is, when I build a pool with the pool preset on the same floor, it’s fully functioning, I just can’t use the big pool duh, very frustrating…
Update:
I didn’t save the functional small pool, and I tried to build the same pool with the same way in the same position, I tried every possible position for the ladders, but strangely the new pool is not functional, is it actually a bug…
Frustrating indeed when the game doesn’t give you any feedback as to why the pool isn’t working. Just a red X isn’t too helpful.
Since adding a standard pool worked once, then failed when you tried it again in exactly the same spot, yes it definitely looks like you’ve hit a bug in how pools operate. I would suggest writing it up as a Bug Report and have your non-working save game available to send to the devs so they can look into it and hopefully determine what’s gone wrong.
Sorry I couldn’t provide any help. 
