save change is not permitted

https://support.microsoft.com/en-my/help/956176/error-message-when-you-try-to-save-a-table-in-sql-server-saving-change

In the Options dialog box, click Designers.
Click to select the Prevent saving changes that require table re-creation check box, and then click OK.
In the Table Designer tool, change the Allow Nulls setting on an existing column.
Try to save the change to the table.

https://stackoverflow.com/questions/6810425/sql-server-saving-changes-is-not-permitted-error-prevent-saving-changes-that

From Save (Not Permitted) Dialog Box on MSDN :

The Save (Not Permitted) dialog box warns you that saving changes is not permitted because the changes you have made require the listed tables to be dropped and re-created.

The following actions might require a table to be re-created:

Adding a new column to the middle of the table

Dropping a column

Changing column nullability

Changing the order of the columns

Changing the data type of a column <<<<

To change this option, on the Tools menu, click Options, expand Designers, and then click Table and Database Designers. Select or clear the Prevent saving changes that require the table to be re-created check box.