I hope this helps someone because not being able to type in a text box was perplexing for a couple of weeks. I had no issues with blog posts. I had no problems creating a blank page and adding a text box.
But I did have issues trying to type in that text block. Grrrr.
The solution, at least for me was to click on Blank Page, Classic Editor.
Just like that, I could type in a text box. Problem solved.
So wanting to add a blank page, don't click on that option at the top of the menu. Scroll down to the bottom of the menu and instead click Blank Page, Classic Editor. See if that solves your struggle.