No free Space on 10.15 even after deleting over 50 GB

sample problem for me. Now, only 400 MB left and cannot clean anything.

OK, I found out.

Try

tmutil listlocalsnapshotdates 

If you see the snapshots list, you can delete like

tmutil deletelocalsnapshots 2019-06-10-232759

I'm not sure what your situation is but you might want to consider backing up and doing a clean install of 10.15, as a suggestion.

Might want to try turning off Time Machine if you have it on. I had that happened, and it had some cached stuff that it marked as free but diskspace reported it as used. not sure if this is your situation or not.

Try this: https://forums.developer.apple.com/thread/117223#362436