The last few days I've been playing with Ubuntu 20.04 under VM and chdkptp works pretty well, both versions, CLI and GUI.
In the latest distribution libreadline.so.6 has been upgraded to libreadline.so.8, this solves the problem:
cd / lib / x86_64-linux-gnu /
sudo ln -s libreadline.so.8.0 libreadline.so.6
sudo ln -s libhistory.so.8.0 libhistory.so.6I have no clue about working on Linux, the last time I worked on a Unix / Linux system was in the early 90s, maybe there is a better solution but I only remembered this. 
Thanks for the report.
Which chdkptp build is this? The 921 snapshot is supposed have readline and the related libraries statically linked to avoid this.
Using symlinks like this isn't recommended since the ABIs are actually different, but I suspect it would work for chdkptp in this case.
The next step is compiling under Linux. 
It should be relatively easy these days
download and configure external libraries with
misc/setup-ext-libs.bash
then make to build. editing config.mk shouldn't be required any more (if you are building in an existing source tree, it would be good to remove any old config.mk and start fresh).
