I'm doing some computation with the population values of the subcatchments, and I'm trying to compute the cumulative values using SUM and then I store it in the user define column, but when I apply the Sum to one of the user defined columns I get this errors.

user_number_2 -used in aggregate function but does not contain anything to aggregate.

Code:
LIST $nodeID STRING;
//Create a list with string values
SELECT SELECTED DISTINCT node_id INTO $nodeID FROM [All Nodes]; 
//Store distinct node values also make it scalar.

LET $i =1;//Variable for the loop 
WHILE $i<=LEN($nodeID);
//start of While loop

DESELECT ALL FROM [All Nodes];

SELECT FROM [All Nodes] WHERE node_id= AREF($i,$nodeID);

UPDATE [ALL Links] SET $link_selected = 0;

UPDATE [ALL Nodes] SET $node_selected = 0;

UPDATE SELECTED SET $node_selected = 1;

SELECT FROM [All Nodes] WHERE $node_selected = 1;
 
UPDATE Subcatchment SET $subcatchment_selected =0;

UPDATE SELECTED SET subcatchments.$subcatchment_selected =1;

DESELECT ALL From Subcatchment; 

SELECT FROM Subcatchment WHERE $subcatchment_selected =1;

SELECT SELECTED (population) INTO $Population_1 FROM Subcatchment; 

SELECT FROM [All Nodes] WHERE node_id= AREF($i ,$nodeID);

UPDATE SELECTED SET user_number_1 = $Population_1 ;
// Moving the Population value to the user define
UPDATE SELECTED SET user_number_2= user_number_1 * 150 / 25;


UPDATE SELECTED SET user_number_3 = SUM( user_number_2); 
 
//Stores values inside the subcatchment

UPDATE Subcatchment SET user_number_1 = 
node.user_number_1, user_number_2 = node.user_number_2  WHERE subcatchment_id = node.node_id;

LET $i=$i+1; 

//Increment
WEND;

// Terminator of the While Loop

DESELECT ALL FROM Subcatchment;