plugin upload error on staging site

Hi there
I've just created a staging site for a site I want to update and change. In the staging version I've activated generatepress and also the premium plugin but I get this error message when I do the latter:

The plugin generated 231 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.

Any suggestions?
Elisabeth