There are 2 different IF commands on FD Robot (I don't know about AX. but I'm guessing it's the same.)
One is: IF V1%=1 THEN 2 ELSE 3, which directly jumps to specified step no.
The other is the one you're showing, where you need an ENDIF after the steps you want to skip if the condition doesn't exist.
IF V1%=1
CALLP[5]
ENDIF
IF V16%>=40
LETVI[V16%,0]
ENDIF