Back in the day there were a lot of hoops to jump through to get an LCCN (Library of Congress Cataloguing Number). One of the hoops involved verifying that the book was not self-published nor were you the author. Also with the old system passwords had to be changed every 90 days which involved counting the days and marking the calendar. Now all of that has changed.
To obtain an LCCN simply click on https://loc.govpublish/prepubbooklink/ and register as a new user either as an author/self-publisher or as a publisher.
If you identify as an author or self-publisher, approval is immediate. Once you create your account you can then apply for the LCCN.
If you identify as a publisher, the process takes a bit longer because your application has to be approved.
And that is it. Thank you Library of Congress for making life much easier.