If you are using a shortcode, you need to specify which calendar to display with it, as described in this section pasted from the documentation:
Displaying a calendar via a shortcode
Starting with version 1.2.0, you can now embed a calendar on any page via a shortcode. Use version 1.2.5 of the PTA Shortcodes plugin to help you generate the proper shortcode. Or use the template below. You MUST specify a calendar to display with the calendar argument. You can use either the numerical ID, or the title, of the calendar you want to display. For example, if you created a calendar that you titled “Main Volunteer Calendar”, the shortcode should look like the following:
[pta-calendar calendar=”Main Volunteer Calendar”]
If you are using the Groups extension, you can also specify which groups to show on the calendar with the “group” argument. See the Groups documentation for more info.