Submit button label is not showing

You must use a button field object instead of a radio button widget to display a text label.

The field properties and individual widget-level attributes don't allow to modify the Font type in such widgets.

Adobe Pi is the standard font type for widgets such as radio buttons and  checkboxes, and it is not used to display a text labels.

It is used instead for displaying the radio button's style, such as a checkmark, a Circle, Diamond, Cross, Square or a  Star when it is pressed or clicked on (found in the options tab of that properties dialogue window).

If you use a button field, make sure that, see the "Options" tab and verify that you select "Label only" for example.