How come I can't flatten this anymore?

Been working on this as I'm still learning the software and I changed something so I can't flatten this anymore.  When I saved it last night I could flatten it but when I opened it this morning it had to rebuild to account for the changes I made.  So apparently I made a change last night that I'm not sure of.  I can unfold this piece by piece but I'd like to know what I did to make it not able to flatten anymore.  Thanks

SolidworksSheet Metal