Multiplication of linear forms not allowed error

I am trying to solve a facility location problem. This is my code:

set S;
param prod{i in S};
param distri{i in S};
param fixed{i in S};
param cap{i in S};
param demand{i in S};

var x{i in S, j in S}, >= 0;
var y{i in S}, binary;

minimize obj :
  sum{i in S} fixed[i]*y[i] + 
  sum{i in S, j in S} x[i,j]*(prod[i] + distri[i])*y[i];


s.t. c1{i in S}:
    sum{j in S} x[i,j]*y[i] <= cap[i];
  

s.t. c2{i in S}:
    sum{j in S} x[j,i]*y[j] = demand[i];

display S;

solve;

printf '\n  Solution: \nMinimum Cost = %.2f\n', obj;

display x;
display y;

data;

set S := 0 1 2 3;

param prod :=
  0 20
  1 30
  2 40
  3 50;

param distri :=
  0 60
  1 70
  2 80
  3 90;

param fixed :=
  0 10
  1 15
  2 10
  3 15;

param cap :=
  0 100
  1 110
  2 120
  3 130;

param demand :=
  0 120
  1 60
  2 70
  3 100;

end;

When I run this .mod file, I get the following error:

example.mod:13: multiplication of linear forms not allowed
Context:  S } x [ i , j ] * ( prod [ i ] + distri [ i ] ) * y [ i ] ;
MathProg model processing error

Here x is the fractional demand provided by facility 'i' for client 'j'.

I removed y[i] from the line mentioned and the error was gone. But if I do that, I get the same multiplication error but this time in c1 constraint.

What is the correct approach? Thank you.

gplk is restricted to linear problems.

It is allowed to multiply parameters and variables. But products of variables are non-linear. You either have to rewrite the model or use a non-linear solver.

Linearization of products involving a binary variable is discussed in this related answer.