Hi Leanah,
First off, here is our article with regards to GneeratePress free theme installation: https://docs.generatepress.com/article/installing-generatepress/
GP Premium: https://docs.generatepress.com/article/installing-gp-premium/
With these said, may we confirm if you’re installing these correctly?
If so, and the error still appears, “kindly check the error logs on your server for a more specific error message, which may give a clue to why this is happening on your website/server. If you need help locating them, please ask your hosting provider to help you with that. If you cannot find error logs or if you don’t find any helpful info in the error logs, enable debugging. See: https://wordpress.org/support/article/debugging-in-wordpress/ . Remember to put things back to normal afterwards.”
Hope to hear from you soon. 🙂