Player sessions are failing to verify
What causes it
Login sessions are being rejected. Causes, in order: Mojang session outage, a stale client session (player needs to restart their launcher), or a proxy set up without proper forwarding.
How to fix it
If it affects one player, they should restart their launcher. If it affects everyone and you run a proxy, fix the forwarding configuration. Otherwise wait out the session-server wobble.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log