Getting an error while trying to update a SQL Table in SMSS?

I'm getting the following error: "Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression". I really don't understand why I'm getting this when I'm only trying to change a value in an existing record. The column that I'm trying to change currently contains NULL. I'm trying to change that value to "All". Why is this happening?

Thanks,