I tried with both the currencyrate and summary plugin. The error message could be more informative...

2021-07-29T20:30:27.590Z DEBUG   plugin-manager: started(1697210) /home/lightning/src/plugins/currencyrate/currencyrate.py
2021-07-29T20:30:27.769Z INFO    plugin-currencyrate.py: Killing plugin: exited before replying to getmanifest

I configured it via plugin=... in config. I also tried the symlink approach, which results in the same error.

I've done pip3 install -r requirements.txt in the /src/plugins/currencyrate which did install a few things.

I'm using Python 3.8.4 via Pyenv, c-lightning v0.10.1rc1 on Ubuntu.

I can call it directly without error:

 ~/.lightning/plugins/currencyrate.py

Hi, it looks like you're trying to run a plugin from the
command line.
...