Can ser cable run underground in rigid pvc conduit

Conduit is used to sleeve SER cable for short distances, and will be oversized. To pull SER cable through a conduit system would require the conduit to be much larger than needed if individual wiring was pulled in, as normally done by electricians.

Why do you need to pull SER cable in underground conduit?

What size SER and conduit?

Installing stand by gen. and would like to run 4/0 ser from ATS 50 ft indoors to a 15 ft 2 1/2 in dia. Conduit run from basement to Gen. pad. I would like to strip the length going into conduit and connect to conduit with a 2 hole clamp connector

See NEC 338.12(A)(2):
You are not permitted to install SER cable underground with or without a raceway.
For underground installations the cables will need to be listed... (typically THWN)