How to change the value of text fields in layers

I'm not sure what the issue is, but this works for me:

function getComp(theName){
	for (var i = 1; i <= app.project.numItems; i++){
		if (app.project.item(i) instanceof CompItem && app.project.item(i).name == theName){
			return app.project.item(i);
		}
	}
	return null;
}
var myComp = getComp("BLOCK_01");
if (myComp){
	var myTextDoc = myComp.layer("R_VAR").property("ADBE Text Properties").property("ADBE Text Document").value;
	myTextDoc.text = "New Text";
	myComp.layer("R_VAR").property("ADBE Text Properties").property("ADBE Text Document").setValue(myTextDoc);
}else{
	alert("Can't find comp 'BLOCK_01'");
}

If you want to create your own tool with my (paid) extension Automation Blocks,

learn here, how to modify things like texts in your project:
https://docs.mamoworld.com/automation-blocks/tutorials-ae/writeCustomTemplateTool

instead of using a custom user interface, you want to read the data from a spreadsheet.
You can learn here, how to work with spreadsheet files in Automation Blocks:

https://docs.mamoworld.com/automation-blocks/tutorials-pr/mogrtFromSpreadsheet

The latter tutorial is for Premiere Pro, but the blocks to work with spreadsheets are identical in both Ae and Pr.