No "Merge" option for merging two calendars

Hi, in the MacOS Calendar, I'm trying to merge one calendar with another. Everything I've looked up (e.g., here) says it's a simple matter of right-clicking the calendar I want to merge, hitting "Merge," and choosing the calendar I want to merge to. I'm getting some wonky behavior, though. I'm usually not getting any Merge option at all (either with the right click, or by looking under the Edit drop-down). Once, though, I did see a grayed-out Merge option on one of the calendars. I've tried to replicated it and can't (even if it's kind of moot if the option is visible but unavailable).

What obvious thing am I missing here? See attached screen grab.

Thanks in advance for any help.

Rob