How to set up an automation to change field in record

I am trying to set up an event capacity automation (under when a record matches condition).

Basically, for a set number of activities (Activities table), I have created individual events. One of the fields in this table is the set capacity number for the event (capacity number). Another field on the table is a linked field of the total number of registrants from the RSVP table on the base (Total RSVPs count). I have another field on the Activities table that is a single-select Open or Closed, which when toggled to open will allow that activity to be viewed on our embedded gallery of available activities for which to register.

I want to set up an automation that triggers the Open/Closed field to be set to ‘Closed’ once the total RSVP number is equal to (or reached) the Capacity number field.

In the automation configuration, I am able to select the Activities table, but I’d like to add the condition that WHEN → “RSVPs” (total count of RSVPs from RSVP table) = “Capacity” (maximum capacity number)
THEN ACTION - the Open/Closed field is set to Closed.

But I am unable to select the “Capacity” field in the dialogue.