"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 enabled the option Prevent saving changes that require the table to be re-created."
Here is the screenshot.
The solution of this problem is to go to Tools -> Options -> Designers -> Table and Database Designers and uncheck the "Prevent saving changes that require table re-creation".
Here is how to solve it: