When I was developing a free plugin to be submitted to WordPress.org repository, I encounter a weird line says
The plugin generated `n` characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
I troubleshoot it by doing trial and error until I found and deleted this last line of php file
Basically PHP code should look like this
<?php /* your code here */ ?>
You can omit the 3rd line and it should work.
This seems to be the culprit, based on my experience, this shouldn’t be worried enough and it’s just a minor WordPress problem, you can contact the plugin author for this error in order to help them developing their plugin flawlessly.