Great job and thanks to the author.
You might want to change cell R17 to the equation below to get rid of the starting error.
=IF(ISERROR(AVERAGE(C3:C55,E3:E54,G3:G54,I3:I54,K3:K54,M3:M54,O3:O54)),"",AVERAGE(C3:C55,E3:E54,G3:G54,I3:I54,K3:K54,M3:M54,O3:O54))