Create a txt file in Extendscript

Give this one a shot.

First you better check if you have permissions to Write Files and Access Network.

Next, you probably need to open fine before writing to it.

var scriptFolderPath = File($.fileName).path; // the URI of the folder that contains the script file    
var TimerFolderPath = scriptFolderPath + encodeURI("/ProjectTimers_sources"); // the URI of the folder for your script resources    
StartButton.onClick = function() {
    var JFile = new File(TimerFolderPath + encodeURI("/File.txt"));
    var content = "That is a text file";
    
    if (!canWriteFiles()) return null;
    writeFile(JFile, content);
    
    // File.write(content);
    alert("File created!");
};
function writeFile(fileObj, fileContent, encoding) {
    encoding = encoding || "utf-8";
    fileObj = (fileObj instanceof File) ? fileObj : new File(fileObj);
    var parentFolder = fileObj.parent;
    if (!parentFolder.exists && !parentFolder.create())
        throw new Error("Cannot create file in path " + fileObj.fsName);
    fileObj.encoding = encoding;
    fileObj.open("w");
    fileObj.write(fileContent);
    fileObj.close();
    return fileObj;
}
function canWriteFiles() {
    if (isSecurityPrefSet()) return true;
    alert(script.name + " requires access to write files.\n" +
        "Go to the \"General\" panel of the application preferences and make sure " +
        "\"Allow Scripts to Write Files and Access Network\" is checked.");
    app.executeCommand(2359);
    return isSecurityPrefSet();
    function isSecurityPrefSet() {
        return app.preferences.getPrefAsLong(
            "Main Pref Section",
            "Pref_SCRIPTING_FILE_NETWORK_SECURITY"
        ) === 1;
    }
}