Suggest to close eM Client and then “go online to your Google Calendar” and “delete the event from there”, and then reopen eM Client.
Failing that “if the event is no longer online” in your Google calendar, but still showing an error in eM Client, then try repairing your Google Calendar.
Lastly if still makes no difference suggest to backup eM Client via “Menu / Backup” and once backed up, remove and re-add your Google account via the automatic email wizard in “Menu / Accounts”.