Yo! Thanks a ton for picking up MasterPlan! I really appreciate it, especially if you have another tool that you were already using.
The clipboard is indeed fully supported for copying and pasting text into and out of MasterPlan on all platforms. It requires an external clipboard package to be installed to work on Ubuntu / Linux, though. Do you have xsel, xclip, or wl-clipboard installed? I thought I remember checking and seeing that one of those should be installed by default on the latest version of Ubuntu.
As for the lack of an error, it's not reporting the error currently as I just now noticed that I am suppressing it; apologies! I didn't really consider why one might not be able to read from the system clipboard. If you do have one of those tools installed and it's still not working (Ctrl+C, Ctrl+V to copy and paste text, just like usual), I'll redirect the error to the log and put out a hotfix build so we can hopefully see exactly what the issue is.