The syntax is a little off. You have to concatenate the combo box value; because it is text, you also need to use the correct delimiters.
Code:
Me.txtPONumber = DMax("[PONumberIndx]", "qryDelivery", "[Purpose] = '" & Me.cboPOPurpose & "'")
Text strings must be delimited with quotes. Expanded, the last part (in red) is: " ' "
I probably wouldn't use DMax() to get this year's PO number - it would depend on what the PO number look like.
What do the PO numbers look like? Is there a special sequence of letters and numbers?
Do you also have a year field in the purchase orders table? You could then do something like
Code:
Me.txtPONumber = DMax("[PONumberIndx]", "qryDelivery", "[Purpose] = '" & Me.cboPOPurpose & "' AND [POYear] = " & Year(Date))