This error message is given when the control variable of a for statement is not a simple variable (but a component of a record, for instance), or if it is not local to the procedure containing the for statement. 

For backward compatibility reasons, it is legal to use a global variable as the control variable - the compiler gives a warning in this case. Note that using a local variable will also generate more efficient code.

program Produce;

var
  I: Integer;
  A: array [0..9] of Integer;

procedure Init;
begin
  for I := Low(A) to High(a) do  (*<-- Warning given here*)
    A[I] := 0;
end;

begin
  Init;
end.
    program Solve;
var
  A: array [0..9] of Integer;

procedure Init;
var
  I: Integer;
begin
  for I := Low(A) to High(a) do
    A[I] := 0;
end;

begin
  Init;
end.