Unable to apply a SAS macro to a column

You most likely do not need to macro code yet.

Macro writes SAS source code before run-time, it does not evaluate data step expressions at runtime.

Learn to write DATA Step code before attempting to abstract it to macro.

DATA Step

This might contain source code statements (the if/then/else) that you want macro to generate

data cars2;
  * calculate;
  if mpg < 20 then varnew = weight + mpg;
  else
  if mpg >= 20 then varnew = weight - mpg;
run;

How would this be abstracted ? Determine which components of the if/then/else would be reused in a different context or with different variables. If you can't determine re-use, don't code a macro.

Consider abstraction #1 (as pseudo-code) that is to generate a complete statement

  if PARAMETER_2 < 20 then RESULT_VAR = PARAMETER_1 + PARAMETER_2;
  else
  if PARAMETER_2 >= 20 then RESULT_VAR = PARAMETER_1 - PARAMETER_2;

or abstraction #2 that is to generate source code for an expression

  ifn (PARAMETER_2 < 20, PARAMETER_1 + PARAMETER_2, PARAMETER_1 - PARAMETER_2)

But why hardcode 20 into the macro ? Why not make that a parameter as well ? If you go that route the abstraction is too much and is templating actual language elements that should be used in a non-macro way. (One might suppose in a purely functional language such as LISP there is no abstraction too much)

Abstraction #1 as macro

%macro calculate(result_var, parameter_1, parameter_2);
  /* generate DATA Step source code, using the passed parameters */

  if &PARAMETER_2 < 20 then &RESULT_VAR = &PARAMETER_1 + &PARAMETER_2;
  else
  if &PARAMETER_2 >= 20 then &RESULT_VAR = &PARAMETER_1 - &PARAMETER_2;
%mend;

data cars2;
  %calculate(varnew,weight,mpg);
run;

Abstraction #2 as macro

%macro calculate(parameter_1, parameter_2);
  /* generate source code that is valid as right hand side of assignment */

  ifn (&PARAMETER_2 < 20, &PARAMETER_1 + &PARAMETER_2, &PARAMETER_1 - &PARAMETER_2)
%mend;

data cars2;
  varnew = %calculate(weight,mpg);
run;