This is meant to be a short info. article for anybody who runs into the same problem I did.
I just had an issue with this where I tried to use the create page button in the middle of the blank Project pages screen. The one that looks like this:
If I tried to use this button to create a page I would get an error that the page could not be created but nothing to explain why.
If you look at the image closely (sorry for the size if it's too small) you'll see that in the upper left is the project I am in. If you look at the upper right corner you'll see the Confluence space that I attached to the project. That is the project space for a different but related project. Note also that none of the pages from the space that is linked show up in the project page space for the project I am in.
After placing a support ticket, I had a momentary flash of an idea. I disconnected the other project space from this one and Voilà, I was now able to create a new page.
I made a suggestion on the support ticket for them to elaborate more on the error message or just let the button do its stuff as expected. Anyway, I hope this helps someone who has this problem.