I am using a SUMIF formula to total checks received from various
clients. I have a list of clients entered into cells A21:A109 as
text.

Rather than copying the text contained in those cells into each
formula I have attempted to use the INDIRECT function to fill that
portion of the argument. Using INDIRECT("A21") within the SUMIF
function I can get the fucntion to reference the text in that cell but
when I try to auto fill the formula down the cell reference will not
change. If anyone has a way to do this I would appreciate the
advice.

I'm attempting to use:
=SUMIF($H$3:$H$6003,INDIRECT("A21"),$O$3:$O$6003)
instead of
=SUMIF($H$3:$H$6003,'Client Name',$O$3:$O$6003)