Basic update of custom field issue

I have seen that in Gantt charts you need a Start Date for each task and if you have time tracking enabled one of your selections is to user Timer Time. If i understood well though this is not correct cause Timer time can be updated every time someone start Timer and stops it. So this is not the correct selection and for this reason i have created a custom field in the project named Start Date.

From this i wanted to have a workflow and on change of an issue from Open state to In Progress state  and if the custom field is not already assigned to Update it with new Date().

SO on transition

I get error that the Start Date field can not be updated.

It seems to me that probably there is an issue with the requirements of the workflow on the code. I really do not know what needs to be exactly there and how and i try to go from examples. I understand that you have to place all dynamic variables that you are using in workflow to be sure that these fields exists on the project but i am not sure exactly on the details.

Also if i remove the requirement though the value is assigned the field in the issue is not updated.

The workflow is the next:

/**
 * This is a template for an on-change rule. This rule defines what
 * happens when a change is applied to an issue.
 *
 * For details, read the Quick Start Guide:
 * https://www.jetbrains.com/help/youtrack/server/2023.1/Quick-Start-Guide-Workflows-JS.html
 */

const entities = require('@jetbrains/youtrack-scripting-api/entities');
const workflow = require('@jetbrains/youtrack-scripting-api/workflow');

exports.rule = entities.Issue.onChange({
  // TODO: give the rule a human-readable title
  title: 'Issue_start_time',
  guard: (ctx) => {
    // TODO specify the conditions for executing the rule
    const issue = ctx.issue;
    
    workflow.message(issue.fields.StartDate);
    if(!issue.StartDate){
    workflow.message("before update");
    workflow.message(issue.fields.StartDate);
      return issue.was('State', 'Open') && issue.becomes('State', "In Progress");
    }
    return false;
  },
  action: (ctx) => {
    const issue = ctx.issue;
    workflow.message('Update should happen');
    issue.fields.StartDate=new Date();
    // TODO: specify what to do when a change is applied to an issue
    workflow.message("After update");
    workflow.message(issue.fields.StartDate);
  },
  requirements: {
    State: {
      type: entities.State.fieldType,
      'Open': {},
      'In Progress': {}
    },
    StartDate: {
      type: entities.Field.dateType,
      name: 'Start Date'
    }
    // TODO: add requirements
  }
});

Also if we comment the last lines where in requirements we declare the StartDate field that is needed. Then 

from the workflow.messages we get

The first 3 messages from the bottom are coming from the guard. "Update should happen" is in action and then after the update you see that the variable has value. But in the issue only the state changes and the "Start Date" is still empty

Is there any other better way to debug ? cause with console log you have to switch tabs to do the state transition and then switch to the workflow edit to update the console to see the log.