A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Try this version:
=IFERROR(IF(VLOOKUP($B9,'WA''s per LI'!$A$5:$G$600,6,0)>=$O9,VLOOKUP($B9,'WA''s per LI'!$A$5:$G$600,6,0)-$O9,0),0)
or, somewhat shorter:
=IFERROR(MAX(VLOOKUP($B9,'WA''s per LI'!$A$5:$G$600,6,0)-$O9,0),0)