want to get highest sales qty and sales person name within specified dates

You can do this without a helper column if you want, with this "array formula" in C15

=MAX(SUMIFS(D2:D11,B2:B11,">="&A15,B2:B11,"<="&B15,A2:A11,A2:A11))

confirmed with CTRL+SHIFT+ENTER

and then this formula (also array entered) in D15

=INDEX(A2:A11,MATCH(C15,SUMIFS(D2:D11,B2:B11,">="&A15,B2:B11,"<="&B15,A2:A11,A2:A11),0))

confirmed with CTRL+SHIFT+ENTER

see attached, I changed the dates to show how it works when you do that.....