This official support FAQ explains the queue rules announced for the Ranked BedWars Beta.

Have I registered?

Complete registration in HellCore's Ranked BedWars Discord registration channel and join the required channel once before queueing in-game. See the official Ranked BedWars guide for the channel references supplied by the team.

Which party system should I use?

Use the RBW Party system from the RBW hotbar item. The default BedWars party system does not work for Ranked BedWars.

Why will a 1v1 match not start?

If both queued players are in the same RBW party, they cannot be matched against one another. One player must leave the party.

Is Player Picking available in 3v3?

The supplied Beta announcement says Player Picking is available only in 4v4. Use an RBW party for 3v3.

What if these steps do not resolve it?

Report your queue mode, party setup, and the message shown by the game. Ranked BedWars is in Beta, so staff may need to investigate a bug.