Mail merge not working as expected with Pages and Numbers. Error "The file couldn't be opened."

System: MacBook Pro, 16in, 2021, M1, Sequoia 15.0.

Apps: Pages and Numbers

What I'm trying to do: Mail merge within Pages using a Numbers spreadsheet to populate an envelope with names and addresses.

What's happening: When I go to mail merge ("Add Merge Field") using a Numbers spreadsheet, I'm met with an error in Pages that says "The file couldn't be opened". This is unexpected because as of a month ago using the same files, I was able successfully mail merge. I can't recall if I had updated my MBP's OS between these time points.

PS. A previous poster mentioned a similar issue, but received no response so I'm making a new post.