Why can't I delete iCalendar account subscription?
Calendars that are part of your Google account cannot in general be deleted from within Calendar.app. To delete the calendar, go to https://calendar.google.com/ (or in your case possibly https://www.focuster.com/ ) and manipulate your calendar subscriptons there. 

Different pieces of software interact with Google calendars using a variety of protocols - Exchanage ActiveSync, CalDAV, http, and maybe others. Some of these protocols do not support all the calendars visible in your Google account, or need to be configured to show calendars beyond those listed as "My Calendars" in the Google web interface.

Information about using Apple's Calendar.app in macOS or iOS with Google Calendars is here:
https://support.google.com/calendar/answer/99358

Using the controls at https://www.google.com/calendar/syncselect you can select the calendars that are visible to Calendar.app - this would allow you to turn off the syncing of calendars you do not want to see (like the "Focuster" one mentioned) without needing to delete them entirely from your Google Calendar account. 

It is possible that a system like Focuster will recreate their calender if deleted or turn it back on for syncing if it is turned off, so you may have to manipulate it through their system rather than just using the Google controls.