I can see the page behind the pop up but I cannot access it. If I hit cancel I go back. If I hit add calendar I get an option to add one of two from F-Droid or Aurora Store.
Actually I looked up fossify calendar by name on my wife’s Brax3 and typed that in first. When it allowed it to work, without even having the app, I just left it with her name…then I did mine with just my name.