A sign in looks like one action, but it runs four separate checks, and each one stops you for a different reason. The wording of the failure tells you which check you are standing in front of, and the three groups need completely different responses.
Reading that wording closely is the fastest step available, so it is written first.
A common misreading is that being dropped when you sign in elsewhere is a fault. Rooms differ in how many devices may hold a session at once, and both behaviours are deliberate.
A reference page and nothing more. No sessions, no accounts and no money pass through here. Sign-in behaviour differs between rooms; the operator's own settings page is authoritative.