As
@Glenntech says the Beta programme is only open to unlocked devices so depending on where your handset was sourced then you may not be eligible, also the Beta programme opens and closes for enrolment so even if your device is eligible then it may be that enrolment has been closed.
The A.13 / OneUI Beta is one of the best I've taken part in and is very stable so official rollout should be on schedule to push next Month