Hello,
Thank you for posting in Q&A forum.
To allow Taskbar customization via GPO, please kindly open GPO editor and navigate to User Configuration > Policies > Administrative Templates > Start Menu and Task bar.
Set "Prevent users from customizing their taskbar" as Not configured or Disabled and save the change.
Once it's applied user will be able to customize the Taskbar.
If there's any further query please kindly let us know.
Best Regards
Zunhui