I'm on the 1.16 snapshot 20w06a, and when trying to sleep I cannot, getting the "You cannot sleep now, there are monsters nearby" message. Going into spectator mode I was able to confirm that there were no mobs nearby. I also got the message in Peaceful difficulty. I was able to sleep in a different chunk, but when I move my bed a few blocks I get the same message. Logging out and back into the world fixed it.