Google-sheets – Searching cells and pulling values in Google Sheets

function dataValidation() {                                                                                     
var ss = SpreadsheetApp.getActiveSpreadsheet();                                                             
var Tr = ss.getRange("'Body Armor'!C1:C20").getValues(); //Testrange
// Variables and sheetnames
        var b = "'Body Armor'!";                                                                        
        var g = "GS256!" ;
        var gC = "'GS256 Classified'!";
        var h = "B10:B27";
        var l = "B28:B36";
// Get DV ranges
        var gh = ss.getRange(g+h) ;                                                                     
        var ghc = ss.getRange(gC+h);
        var gl = ss.getRange(g+l);                                                                      
        var glc = ss.getRange(gC+l);
// Build DV rules
        var ghr = SpreadsheetApp.newDataValidation().requireValueInRange(gh,true);
        var ghcr = SpreadsheetApp.newDataValidation().requireValueInRange(ghc,true);
        var glr = SpreadsheetApp.newDataValidation().requireValueInRange(gl,true);
        var glcr= SpreadsheetApp.newDataValidation().requireValueInRange(glc,true);
// Loop through each cell
        for(i=0;i<Tr.length;i++) {
                var j = i+1;
                var Hr = ss.getRange(b+"H"+j);
                var Jr = ss.getRange(b+"J"+j);
                var Lr =  ss.getRange(b+"L"+j);
// Test the test range and apply corresponding DV
        if (Tr[i][0] == true) {
                Hr.setDataValidation(ghcr);
                Jr.setDataValidation(ghcr);
                Lr.setDataValidation(glcr);
                }
                else {
                Hr.setDataValidation(ghr);
                Jr.setDataValidation(ghr);
                Lr.setDataValidation(glr);
                }
        }
}

The problem is data type inconsistency: text vs numeric. Your formula

=IF(LEN($A$2)>0, MID($A$2,LEN($A$2),1), 0)

returns a string "7". Your lookup table has number 7. No match.

A way to convert strings to numbers is to add 0:

=IF(LEN($A$2)>0, 0 + MID($A$2,LEN($A$2),1), 0)

However, I understand you are working with base-36 numbers, where digits can also be A, B, C, and so on. in that case, the above would not work for those digits. Instead, to have consistent data type you need to convert column E entries to text.

The easiest way to put "7" in a spreadsheet cell is to precede it by a single apostrophe: enter '7. The apostrophe will not be a part of cell content, it only designates the type of cell content as a string.

One can also perform conversion with =text(7, "0")