Here is the link to the archive.
https://downloadarchive.documentfoundation.org/libreoffice/old/
I do not know why the newer versions not work on your Mac, and I do not know which version will work.
My tips:
- try to install a very old version (6.x.y, 5.x.y, 4.x.y). If it not work, then the root of the problem is located in your Mac.
- try to search in the Forums and on the Ask site:
is the latest version compatible with Mac OS Catalina 10.15