Hope it helps

Only the designated approvers can validate it. In our system, approval is allowed only for:

• The Timesheet Manager assigned to the employee

• The employee’s Manager

• The Project Manager of the related project

• Or the user who created the timesheet
  
  The visibility of the Validate button is controlled by the following method:
  
  @api.depends_context('uid')

• def _compute_can_validate(self):

•     is_manager = self.env.user.has_group('hr_timesheet.group_timesheet_manager')

•     is_approver = self.env.user.has_group('hr_timesheet.group_hr_timesheet_approver')

•     for line in self:

•         if is_manager or (is_approver and (

•             line.employee_id.timesheet_manager_id.id == self.env.user.id or

•             line.employee_id.parent_id.user_id.id == self.env.user.id or

•             line.project_id.user_id.id == self.env.user.id or

•             line.user_id.id == self.env.user.id)):

•             line.user_can_validate = True

•         else:

•             line.user_can_validate = False
  
  Also you can found to set timesheet manager based on employee :-
  

•