The March 30 Dota 2 Update temporarily disabled the All Pick mode in Ranked Match on Dota 2. When you tried selecting All Pick, a pop up message appear saying “For the next few days, only Captain’s Mode and Random Draft and will be available in Ranked Matchmaking“.
We don’t have confirmation yet why valve disabled the All Pick in Ranked Match, but it might be an experimental to collect data and trends.
For now, you can still play Ranked Match on Captains Mode or Random Draft..