Error

I had thos plugin installed but deleted it because I was seeing errors in the backend.
I reinstalled minutes ago but got this after installation:

The plugin generated 203 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.

Notice: Trying to access array offset on value of type bool in /home/(site-name)/public_html/wp-content/plugins/subscribe-to-category/subscribe-to-category.php on line 28