Can't delete a sync calendar which is not needed anymore.

Hi All,

I can't remove a sync cal created by mistake as per: https://partner.booking.com/en-gb/help/rates-availability/extranet-cale…

I have a calendar sync called "Unknown" which was tentativeley sync "7 minutes ago" for which booking web site says "Import needed" with the button "Complete setup"

Can't get the button remove like I have if the sync passed to "Ok" in the past.