First off, I am really sorry to hear you had a tough time getting it to work.
I just tested it with the Twenty Sixteen version 2.8. and WordPress 6.1.1 and it works fine. It will remove the copyright text if the code is entered correctly.
We offer free support for the plugin and we wish you would have posted a support thread here so we could have helped you with this situation. Would you like to do that now?
Please try adding the code below in Tools > Remove footer credits > step 1:
<a href="https://wordpress.org/" class="imprint">
Proudly powered by WordPress </a>
Leave Step 2 empty.
We spent a lot of time developing and supporting this plugin for free and it can be discouraging to see such a review. Especially since we just tried it on our end and it works fine with that theme.
Please don’t hesitate to reply and we will do our best to help out and get to the bottom of this asap.