SSMS may give you this message when saving changes made via the table designer GUI:
Saving changes is not permitted. The changes you have made require the following tables to be dropped and re-created.
You have either made changes to a table that can't be re-created or enable the option Prevent saving changes that require the table to be re-created.
By default this is not enabled, to prevent you from accidentally dropping tables, but if you're sure you want it enabled, here's how: