axiom
-- m:(FLOAT,FLOAT,INT) -> FLOAT
m(L, r, n) ==
  r := 1 + ( r / 12)
  ss := eval( series(1/(1-'r)), r)
  s0_n_1 := ss.(n-1)
  result := (L * (r ** n))  / (1 + (r * s0_n_1 ))

axiom
m( 240000, 0.055, 300)

   There are 10 exposed and 4 unexposed library operations named eval
      having 2 argument(s) but none was determined to be applicable.
      Use HyperDoc Browse, or issue
                              )display op eval
      to learn more about the available operations. Perhaps
      package-calling the operation or using coercions on the arguments
      will allow you to apply the operation.
   Cannot find a definition or applicable library operation named eval
      with argument type(s)
                                     Any
                                    Float

      Perhaps you should use "@" to indicate the required return type,
      or "$" to specify which version of the function you need.
   FriCAS will attempt to step through and interpret the code.

(1)

axiom
m( 240000, 0.05367, 300)

(2)