Sorry, this is not an answer but I'm lacking the rep to comment.
I'm one of the author's of this plugin and will need additional information to help debug. Can you please link me to the documentation you were following to get to this step, and any additional context that led up to this stack trace?
Just a guess, but this appears that you've enabled the UpwardConnector module without configuring a path to an UPWARD file, which is required. To remedy that you would log into your backend and configure the path to that file in Stores > Configuration > General > Web > UPWARD PWA Configuration.