Same problem here. It doesn’t show up in 4.25, nor in 4.26.
I found this UE-85966 issue report — “USD Stage is not available on Mac and Linux” which is already flagged as Fixed in 4.26, but I compiled master branch yesterday and USD Stage manager is still missing from the Window menu. Maybe those commits fix this problem only for Macs? That I wouldn’t know, as I’m on Debian Bullseye.
I’m very eager to test out Epic’s USD implementation with USD assets exported from Houdini.
Anyone got the USD Stage to show up on GNU/Linux?