Don't create formulas in column validation on the PrjStatus column, because the formula cannot refer to another column.
You can create list validation.
Navigate to list settings->Validation settings->create the following formula->save:
First, as mentioned in the comments above, you should not assume that
ID of the first item is 1, you need a more predictable way to get
Second, the set_item method should be called from the
variable that holds the reference to the item you retrieved. You
code calls listItem.set_Item, but listItem is not a defined
Finally, the attempt to ...