I am having the same problem. I have uninstalled and reinstalled half a dozen times since yesterday. I am using this plugin on all my StudioPress themed sites but yesterday I got that bad header notice and it won’t work, even when I activate from the plugins panel. I tried the theme community forum to no avail.
The environment is as follows: WordPress 5.4.1 running Lifestyle Pro theme. Using Genesis 3.3.2.
I had downloaded a version of Genesis Framework in February this year and WP notified me of the newer version which I installed immediately but still no Simple Hooks.
I left it overnight because this old large blog might have needed extra time to digest all the changes. I upgraded PHP from an old 5.6 to 7.1. I also was converting the domain to use https: protocols but that was all complete when I tried to install. Right now, not even the WP dashboard is functional. I hope there is another workaround. I do not have a disable Gutenberg plugin though I did add on the Classic Editor plugin but that was after all the activation & delete cycles.
Today I have just downloaded a .zip version from WP.org Plugins and will ftp that this afternoon and try again. IF that fixes it I will let you know. I am lost without its functionality to add custom features.