Decision variable not allowed CPLEX model

Question

 int NStations1= ...; int Nstations2=...;

range Stations1= 1..NStations1; range Stations2= 1..Nstations2;

int demand1[Stations1]=...; int demand2[Stations2]=...; int 
distance[Stations1][Stations2]=...;

int AvailTime=...; int Capacity=...;

dvar float+ x[Stations1][Stations2]; dvar float+ f[Stations1] 
[Stations2];

 minimize sum(i in Stations1) sum(j in Stations2) x[i][j]*distance[i] 
[j];

 subject to {

 forall (i in Stations1) sum(i in Stations1, j in Stations2) (f[j][i]- 
f[i][j]) <= demand1[i];

 forall (i in Stations1) sum(j in Stations2) x[i][j] == 1 ;

 forall (i in Stations1) sum(j in Stations2) x[j][i] == 1;

 forall (i in Stations1) 0<= f[i][j] <= x[i][j]*C;

QUESTION: I get the error in the last line, the x[i]*[j], times C, that's where he gives the error of decision variable not allowed

DATA FILE:

NStations1=5; Nstations2=5;

demand1=[5 , 3, 10, 7, 8]; demand2=[5 , 3, 10, 7, 8]; distance=[ [0 7 2 3 4 ] [ 2 0 2 3 3 ] [1 4 0 2 1 ] [7 3 1 0 3 ] [4 7 5 3 0 ] ]; AvailTime=3600; Capacity= 70;

Answer 1

If you add

int C=10;

and turn

forall (i in Stations1) 0<= f[i][j] <= x[i][j]*C;

into

forall (i in Stations1, j in Stations2) 
 {
   0<= f[i][j];
    f[i][j] <= x[i][j]*C;
 }

your model will work much better

NB:

if x,y,z are decision variables x<=y<=z is forbidden and should be turn into x<=y and y<=z