Two options:

1]

=IF(I6<"",LOOKUP(I6,Index!$C$8:$C$60,Index!$D$8:$ D$60),"")

2] Add a record in C7:D7 consisting of 0 and ="" on Index without
disturbing the ascending order that is applied to C8:D60.

This allows you to invoke:

=LOOKUP(I6,Index!$C$7:$C$60,Index!$D$7:$D$60)

allowing to avoid #N/A for a lookup ref that is empty.

Killer wrote:
=LOOKUP(I6,Index!$C$8:$C$60,Index!$D$8:$D$60)

Im using this code on my excel sheet and when Column I6 is blank it shows
N/A in column M6 how can this be removed not to show the N/A

Thanks ahead!