Hi,
To know the issue more clearly, would you please tell more information about the following questions:
1, Did the issue happen on every GPO or the specific GPO?
2, Did all the DCs have the same issue?
3, Which user did you used to edit the GPO?
Following link for your reference: https://social.technet.microsoft.com/Forums/en-US/6c5e2632-67f4-4c8f-952c-66e5b2cdfe37/cannot-set-account-policies-password-policies?forum=winserverGP
Best Regards,