I don’t know the secret sauce, but from what you say you are doing this delete on the icloud web site with a browser. Do you get the same result when you use the Calendar app on either or both Mac or iPhone/iPad?
I don’t get the option to delete a calendar on IOS. That button is missing. Same on my mac. I have one other calendar which is shared between my family members. If I access that calendar on IOS I do see the delete button. Thank you for your suggestion though.
I believe you have solved my issue. My Bill Planner calendar is my only calendar on my icloud account. The other calendar, our family one, is shared from another Apple ID and is the only one I use.
Thanks.