Try This.

select mytable
if empty (variable1)
scan for field2 = variable2
Endscan
Else
scan for field1 = variable1
Endscan
Endif

>HI
>
>I am not succeeding by using the structure below:
>Looks like case or if does not match with scan.
>
>==========================================================
>select mytable
>
> if empty (variable1)
>
> scan for field2 = variable2
>
> else
>
> scan for field1 = variable1
>
> endif
>
>
> endscan
>===========================================================