When making some datatable changes in db diagram environment, a popup window might show up with this message:
Saving changes is not permitted. The changes that 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./p>
meanwhile saving progress stops.
It is because that SSMS by default prevents some operations that requires target tables to be recreated. To go over this block, the highlighted item in Options window needs unchecking.
For more details refer the official knowledge base link:
Error message when you try to save a table in SQL Server 2008: "Saving changes is not permitted"