Showcase has its OWN Prefix System that is separate from Thread Prefixes. What MIGHT be happening is that you have the "Require" option set on one or more of your Showcase Category's via the Prefix Tab settings...
Edit each of your category's and check the Prefix Options Tab settings and make sure you don't have the option marked with the RED ARROW checked.
NOTE: the TITLE of that is wrong... it should say "Default Item Prefix" (this is fixed in the next version).
View attachment 1832