Conditional Running Tally / Cumulative Sum: Looking for Formula or Script - sql

Question

I work in a warehouse, and I'm using Google Sheets to keep track of inventory. Adding and subtracting is easy, but I've been tasked with creating a "reserve" system:
A number of pieces of stock are reserved for upcoming jobs. When that stock is ordered and received, the reserve quantity is "satisfied" and decreases by the number of pieces received. The problem with setting it up just like the ADD and SUBTRACT function is that not all stock received is "reserved", and my RESERVE totals end up being "-57", "-72", "-112", etc.
I have a large dataset of form responses logged in four columns: Timestamp, Item ID#, Action (ADD, SUBTRACT, or RESERVE), and QTY. What I'm looking for is a way to create a running tally in column E for each unique Item ID#, using the values in column D "QTY". I need for any value <0 to return "0".
Example Sheet
I've been able to create a running tally formula that satisfies my conditions for one Item ID# at a time. To avoid creating a separate column for each Item ID#, though, I need to figure out how to apply it separately to each unique Item ID#, and array it down Column E so each new form response is calculated automatically.
=if(C2="RESERVE",E1+D2,if(and(C2="ADD",(E1+D2)<0),0,E1+D2))
The closest thing to a solution I've been able to find is a script created by user79865 for this question titled: "Running Total In Google Sheets with Array". Unfortunately, trying to plug this into Google Sheets Script Editor gives me an error popup:
TypeError: Cannot read property "length" from undefined. (line 2, file "runningtotal")
I have no programming background and never dreamed I'd be looking at code just to make a running tally.
If anybody can offer any insight into this, fixing or replacing the script or offering an ARRAYFORMULA solution, I'd really appreciate it!
function runningTotal(names, dates, amounts) {
var sum, totals = [], n = names.length;
if (dates.length != n || amounts.length != n) {
return 'Error: need three columns of equal length';
}
for (var i = 0; i < n; i++) {
if (names[i][0]) {
sum = 0;
for (var j = 0; j < n; j++) {
if (names[j][0] == names[i][0] && dates[j][0] <= dates[i][0]) {
sum = sum + amounts[j][0];
}
}
}
else {
sum = '';
}
totals.push([sum]);
}
return totals;
}