VBA For Error Message If Value Is Referenced In Sheet

I have a worksheet called "PurchaseOrders" and a worksheet called "Orders."

In the "Orders" Worksheet I have a command button. When this button is clicked it writes to cells in the "PurchaseOrders" Worksheet. The cell range it writes to is N21:N1000. It basically writes an invoice number which exists in cell B6 of the "Orders" Worksheet with each number listed in order.

After it has written, when I return to the "Orders" Worksheet, if the button is clicked again but the PurchaseOrders" Worksheet N21:1000 contains an invoice, the same as "Orders" Worksheet B6 (a duplicate), I would like an error message to be returned stating "Please Create a new invoice." I already have a sub where error messages are recorded for void fields and unregistered customers.

How would I write an if statement in this sub to say:

IF Cell range in "PurchaseOrders" Worksheet (N21:N1000) does not equal the code in "Orders Worksheet (B6) please process code as normal. Else if it does please display an error message and do not proceed with processing any code? For some reason I'm having trouble naming the cell range.