'You cannot add or change a record because a related record is required in table 'Booking'.' error when removing a PKFK value in a SQL database in C#
&quot;UPDATE Room SET BookingID = '', Occupied = False WHERE Room.BookingID = '&quot; + TxbID.Text + &quot;'&quot;;

You don't have a Booking record with a BookingID of ''. I suspect you have your relationship the wrong way round; a Booking should have a RoomID rather than a room having a BookingID, or if you want to persist with the current architecture you should perhaps be setting Room.BookingID null
Please read this website, not because it solves your problem, but because you're making the most grave mistake anyone can make when writing database software, and you need to kick the habit now. (If you did it while working for my company, they'd just fire you)