Can't add relative quantities

You have specified an expression that attempts to add together two
addresses, which is a meaningless operation. For example,

   ABC   DB  ? 
   DEF = ABC + ABC ;error, can't add two relatives 

You can subtract two relative addresses, or you can add a constant to a
relative address, as in: 

   XYZ    DB  5 DUP (0) 
   XYZEND EQU $ 
   XYZLEN = XYZEND - XYZ       ;perfectly legal
   XYZ2 = XYZ + 2              ;legal also