Hi Stanley,
I am Dave, I will help you with this.
There is no separate setting for the time format on that clock app or its notifications for 24/12 hour format.
Open the old Control Panel, then at the top right, set View to icons.
Open Region, and on the resulting dialog, click 'Additional Settings.
On the next dialog, select the Time tab, is both the short and long time formats set to the 24 hour format?