don't think it's free because it isn't

How do you know that the error wasn’t yours? Why didn’t you try to contact the plugin’s author about this by posting a topic in the plugin’s dedicated sub-forum?

Try reviewing:
https://pippinsplugins.com/how-to-leave-a-good-bad-review/
https://chrislema.com/theres-wrong-way-give-plugin-feedback-wordpress-org/