I'm a bit late, but I was having the same problem and was able to figure out a solution
/portfolio —> parent page
/portfolio/fine-art —> wanted this URL for a separate subpage, but was getting that error
I found if I temporarily changed /portfolio to another name, I could change what was previously /fine-art to /portfolio/fine-art and put /portfolio back to its original URL just fine. I guess it's about the order of implementing things.
Might also work to temporarily disable the parent page, but haven't tried that.
