AMPL adding min max constraints to items in set - optimization

Question

I have recently been introduced to AMPL in a class and I am currently working on an optimization problem that requires me to find the minimal cost for the demands required. The actual lines I have in question are these:
1. This is in my model file:
minimize Total_Cost:
sum{i in GENS, j in LOADS} cost[i,j] * Allocate[i,j];
subject to GenConst {i in GENS}:
sum {j in LOADS} Allocate[i,j] <= Generation[i];
subject to DemConst {j in LOADS}:
sum {i in GENS} Allocate [i,j] >= Demand[j];
in my data file:
param: GENS:
GenerationMin GenerationMax := #defines set "GENS" and param "Generation"
GEN1 10 90
GEN2 10 100
GEN3 5 85 ;
We have only ever worked with having problems where our demand=generation, but none with having minimum, maximum along with demand != supply. I get the following error when running my data file within ampl GenerationMin is not a subscripted param . When running this script with only the max value it runs fine. The issue, and I am only guessing, are with the lines above. Could someone explain to me where I am going wrong and how to fix this issue?
EDIT: I can include all of my code, although in case anyone wants to reproduce the problem. .mod file:
set GENS;
set LOADS;
param Generation {GENS} >=0;
param Demand {LOADS} >=0;
param cost {GENS, LOADS} >= 0;
var Allocate {GENS, LOADS} >= 0; #{GEN1, LOAD1}, {GEN1, LOAD2... etc}
minimize Total_Cost:
sum{i in GENS, j in LOADS} cost[i,j] * Allocate[i,j];
subject to GenConst {i in GENS}:
sum {j in LOADS} Allocate[i,j] <= Generation[i];
subject to DemConst {j in LOADS}:
sum {i in GENS} Allocate [i,j] >= Demand[j];
.data file:
data;
param: GENS:
GenerationMin GenerationMax := #defines set "GENS" and param "Generation"
GEN1 10 90
GEN2 10 100
GEN3 5 85 ;
param: LOADS: Demand := #Defining set "LOADS" and param "Demand"
Load1 70
Load2 20
Load3 30
Load4 60;
param cost:
Load1 Load2 Load3 Load4 :=
GEN1 39 14 11 14
GEN2 27 9 12 9
GEN3 24 14 17 13;
option solver cplex;
solve;
display Allocate;
display Allocate, Total_Cost > Output.txt

Answer 1

The correct AMPL syntax is:
set GENS;
param GenerationMin {GENS} >=0;
param GenerationMax {GENS} >=0;
data;
param: GENS:
GenerationMin GenerationMax :=
GEN1 10 90
GEN2 10 100
GEN3 5 85 ;
display GENS,GenerationMin,GenerationMax;
i.e. use both GenerationMin,GenerationMax in the model and in the data section.