Saving changes are not permitted. Because the changes you have made require the listed tables to be dropped and re-created
If you modify table in Microsoft SQL Server Management Studio, it sometimes says something like
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 recreated or enabled the option Prevent saving changes that require the table to be re-created.
Do you know where mentioned option is hidden?
Tools -> Options -> Designers -> Table and DAtabase Designers. Find there checkbox Prevent saving changes that require table re-creation, remove checkbox and enjoy results.