To enable CoachAccountable to connect to your Apple Calendar for the purpose of syncing, you'll need to make a password specifically for [systemName] to have that access. (Which is nice, because that way you don't have to share your actual Apple account password!)
To do so, visit your Apple Account management page.
Under the "Security" section, if you haven't already enabled Two-Factor Authentication, you'll find the place to do so. (Unfortunately this is a necessary step that must be complete BEFORE Apple allows you to create an app-specific password.)
Once Two-Factor Authentication is enabled, the Security section of this page will be changed to look more like this:
Under "App-Specific Password" click the "Generate password..." link and you'll be on your way. Create one for CoachAccountable and you'll have what you need to set up the calendar sync with your Apple Calendar.