"Atlas, At Last" in Borderlands 3 is a main story mission that can't technically be missed. Its chapter 9 of the main story. The plot points in the mission follow going through
Its recommended to be at around level 17 for this section of the game.
Check out this IGN guide over this section. Its very helpful and informative. https://au.ign.com/wikis/borderlands-3/Atlas
The hyperlink for the page does have a comma in it however, that is actually part of the URL.