Adding/removing issues from boards based on assignee

I want to have a custom workflow rule that automatically assigns issues to a board based on the assignee's name. The rule is triggered whenever the assignee field is updated. However, I'm encountering an errors.

The expected behaviour: 

When the assignee field is updated, the workflow rule should check the assignee's name and assign the issue to the corresponding board. For example, if the assignee's first name is "John" the issue should be added to the "John schedule" board.

Here's the code I wrote with the help of ChatGPT, if it helps anything.

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

exports.rule = entities.Issue.onChange({
  title: 'Change-assignee-board',
  guard: (ctx) => {
    // TODO: specify the conditions for executing the rule
    return true;
  },
  action: (ctx) => {
    const issue = ctx.issue;
    const assignee = issue.fields.Assignee;

    if (assignee !== null) {
      const firstName = assignee.fullName.split(' ')[0];
      const boardName = `${firstName} schedule`;

      issue.addComment(`Adding issue '${issue.id}' to the board '${boardName}'`);

      const board = ctx.project.boards.find(board => board.name === boardName);
      if (board !== null  && !issue.isInProject(board.project)) {
        board.addIssue(issue);
        issue.addComment(`Issue '${issue.id}' added to the board '${boardName}'`);
      }
    } else {
      const previousAssignee = issue.fields.becomes(ctx.Assignee);
      if (previousAssignee !== null) {
        const previousFirstName = previousAssignee.fullName.split(' ')[0];
        const previousBoardName = `${previousFirstName} schedule`;

        issue.addComment(`Removing issue '${issue.id}' from the board '${previousBoardName}'`);

        const previousBoard = ctx.project.boards.find(board => board.name === previousBoardName);
        if (previousBoard !== null && issue.isInProject(previousBoard.project)) {
          previousBoard.removeIssue(issue);
          issue.addComment(`Issue '${issue.id}' removed from the board '${previousBoardName}'`);
        }
      }
    }
  },
  requirements: {
    // TODO: add requirements
  }
});