I had this error when installing the pro version on my test site. I solved it by:
1 Uninstall Generatepress theme by installing another temporary theme. This allows you then to delete the now inactive Generatepress theme using the tiny ‘Delete’ link at the bottom right of the screen
2 Exit the WordPress application
3 Restart the WP application and install Generatepress as shown in the installation video (ie, using the supplied .zip file). This will then install the theme
I couldn’t find these hints on the forum so forgive me if I am repeating something already solved.