Dear Nigel,
Welcome to the forum here.
As there is no output after the second HTTP call showed in the screenshot above, please check whether you added user A in your tenant to the site Test Library in your environment. If no, I suggest you add it, update outputs('Send_an_HTTP_request_to_SharePoint_2')?['body']?['d']['results']?[0]['Id'] in the Set variable connector and then run the flow again to see the result.
Welcome to share any updates when you have time if you need further help on this issue.
Thanks for your effort and time!
Sincerely,
Cliff | Microsoft Community Moderator