I thought that Leo install problems on Linux had been solved, but apparently not so. I tried to install Leo in Linux Mint to a new virtual environment. It failed to install because it wanted versions of PyQt and PyQtWebengine that don't seem to be available for Linux.
In recent past, it had been found that one had to install older Leo Leo versions, 6.4, or even 6.1, before upgrading to later versions. Today I couldn't get 6.4 to install. The reason turned out to be that PyPi apparently doesn't have the right linux versions of PyQt5 and PyQtWebEngine.
I was able to get Leo installed in two different virtual environments by first installing the PyQt packages. The only combination I was able to install successfully was:
PyQt5 (5.14.0)
PyQtWebEngine (5.12)
All other versions failed for one or the other of PyQt5 or PyQtWebEngine. I had to pip-install these first. Then I pip-installed Leo with success, getting version 6.5.
Apparently you can't get a list of what versions PiPi has available. The pip search function doesn't work anymore, and the old way of installing with a blank version (e.g., pip install PyQt5==) to get a list of available versions doesn't work either. If you search on the PyPi web site, it tells you there are versions which don't install and does not say that it has versions that actually do.
I don't know what the solution is here, but it could be a nasty problem for new users on Linux.