Fixing a specific cell and copying over

hi guys

please take a look at the formula I have below

=IFERROR(IF(LOOKUP(2,1/SEARCH($AG$4:$AG$15,$X21)),"T"),"O")

the part of the formula I need looked at is the ,$X21)) is there a quick way I can copy this formula down so it goes from x21 to x22 and so on until it gets to x5000 then add $x$21 so the cell is locked.

I have tried to put $x$21 the copy down however the cell number doesn't change

thanks for your help

Simon