I wanted to post a followup here:
This was not a GeneratePress issue.
I had someone test the site on a different server and everything worked fine after changing the permalink setting to “Plain.”
Yesterday, I tried to edit a post or page by adding in an image; I got the same result “Updating Failed.” I could add in text but not images.
This morning, I ran the Site Health Checker in the Dashboard and there were other errors in the firewall regarding ip address. I fixed this.
Right now, I can now update the posts & pages, as well as add new content.