I have a flow I built that takes input from a MS Form, and uses two variables to save an element of the submission to a specific folder in a document library. I.e. if the respondent said they live in a certain state, I have an Object variable that maps 'Alabama':'Southern Region', and so on...

and the subsequent variable takes the specified state input from the form and assigns the correct region:

variables('varRegions')?[outputs('Get_response_details')?['body/rf0c4529364ef42d18522441d560f9fad']]
And it works great. But now I'm trying to do something similar taking values from a Calendar Event, and using the first variable to map "Categories" (aka "tags") assigned to the event, and mapping those to a broader category group:

and then use the subsequent variable to grab the actual category for a specific event for use later in the flow (specifically, adding that value to a column in a SharePoint list):

variables('varSubCategories')?[outputs('Get_calendar_view_of_events_(V3)')?['body/categories']]
But when I run the flow, I receive this error message:
Unable to process template language expressions in action 'Initialize_variable_Categories' inputs at line '0' and column '0': 'The template language expression 'variables('varSubCategories')?[outputs('Get_calendar_view_of_events_(V3)')?['body/categories']]' cannot be evaluated because property '' cannot be selected.
What am I doing wrong?