Are you certain that is the exact message?
I generally don't install non-standard addons, but to test, I installed this one and tried to load the sample.capx.
I got an error message that said 'the project uses addons that are not installed'. It states that I need another of Rexx's addons called 'Timeline'. Is this the same for you? If so, just download from HERE: https://rexrainbow.github.io/C2RexDoc/plugins.md/rex_timeline.html. I did and installed in the 'Plugins' folder - NOT a Behaviour!
The sample will then work.
This is why I don't install non-standard addons - especially Rexxrainbow's - because they usually require even more addons to get the functionality you need.