If you change a table in the SQL Server Management Studio designer and try to save, you may get this:

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.

This is not a bug and it is not your permissions. It is a safety setting, and it is on by default.

Certain changes cannot be applied to an existing table in place. Adding a column in the middle rather than at the end, changing a column's data type, or altering the null setting all require SQL Server to build a new table, copy the data across, drop the old one, and rename. The designer is perfectly willing to do that for you, which is exactly the problem: on a large table it is slow, and it happens without you necessarily realizing the table is being rebuilt underneath you.

To turn the guard off, go to Tools, then Options, then Designers, then Table and Database Designers, and uncheck Prevent saving changes that require table re-creation.

Before you do, understand what you are turning off. The setting exists to stop you from accidentally triggering a full table rebuild in a graphical designer. On a small development table, unchecking it is harmless and saves you a headache. On a large production table, it is worth stopping to think, because a rebuild means copying every row and holding locks while it happens.

My own habit is to leave the setting on and write the ALTER TABLE statement myself instead. Not because the designer is wrong, but because when the change is scripted I can read exactly what it is going to do, run it in a transaction, and check it into source control with everything else.