Form not sending – properties of undefined (reading ‘then’)

The form is giving feedback to say a message is sent, but nothing comes through.

I;ve tried sending to multiple different emails, all with different domains.

I’m on the latest version, and don’t have any caching plugins installed. It’s multi-site.

I’m seeing the following error in console:
Uncaught TypeError: Cannot read properties of undefined (reading ‘then’)

Any clues as to what the issue might be?

The page I need help with: [log in to see the link]