Never fails on a new install of Sql Server Management Studio that I forget to go in an cut off preventing of save changes warning before I start creating tables.
So just a reminder (and also so I don't google it again because I always forget it is under 'designers'), when you get this error:
Saving changes is not permitted. The changes that you have made require the following tables to be dropped and re-created.
Just go to Tools -> Options -> Click 'Designers' (should put you into Table and Database Designers Sub Menu)
And uncheck: Prevent Saving changes that require table re-creation.
Love those hidden toggles.