Error 'Unknown WBS' when entering timesheet in Manage My Timesheet Application

I am entering timesheets using 'Manage My Timesheet' application for a work package to which I am staffed to.

However, when I enter time and save, error message prompts 'Unknown WBS element.

Note - Personnel Number is active. Employee's employment duration and Work Pacakge dates are aligned.

When I used 'Notify missing timesheet application' I am able to post timesheet successfully without any errors.

Please assist

Thanks