Cannot Create Series "Not a Valid Choice: 6"

Good morning,

Whenever I attempt to create series, I am given the message at the bottom of my screen "Not a valid choice: 6". I attempted this on my PC using Google Chrome, as well as on my iPhone's Safari. I double checked and I don't see anything that I would have entered in to be invalid. I did not exceed the allotted amount of "themes" or "art styles".

Any help is greatly appreciated.

Andrew