I need a ‘facepalm’ category apparently. There’s been a lot of that lately.
You cannot import, let-alone work on, Photoshop files (“PSB” file extension) in Lightroom. It flat-out doesn’t support them. That’s such a weird limitation & oversight, for a program that Adobe now officially names ‘Adobe Photoshop Lightroom’.
See also this six year old thread on Adobe’s website, asking for this feature.