Installing Plone 5.2.1 on Ubuntu 20.04 is possible, but requires at least one extra dependency that is not one of the normal dependencies for earlier versions of Ubuntu.
As a test case, I installed Plone 5.2.1 using the Unified Installer on the most-up-to-date version of Ubuntu 20.04 (as of April 8, 2020). Initially, I included the following dependencies before attempting an install:
sudo apt install libz-dev libjpeg-dev libssl-dev libxml2-dev libreadline-dev wv python3-pil-dev libxslt1-dev
... and then ran the install.sh script. It failed because a particular header file (ffi.h) was not found. Consequently, I added:
sudo apt install libffi-dev
... and re-ran the install.sh script. Success!
For the moment, we need to note to future installers on Ubuntu 20.04 to add libffi-dev to the list of dependencies. For your convenience, that list is now:
sudo apt install libz-dev libjpeg-dev libssl-dev libxml2-dev libreadline-dev wv python3-pil-dev libxslt1-dev libffi-dev