Hi Michael,
I guess you are trying to copy a screen of the app that I created here! I also faced the same issue and I can copy only the Login screen and Add Items screen! I cannot copy the Checkout screen and Returns screen!
Seems like I found the issue! I tried to remove the List in the checkout screen and then copy the screen and paste it and wow it worked! Then I removed the list and made that whole group as a list again but no luck!
Then I cut them down to two groups and they both are two lists and then I grouped that two lists and made a group and FINALLY IT WORKED! 
I think the issue maybe the grouping! You need to clone the app again to check this!
Thank you