Error code 273 means the same account launched the experience from a different device while you were already in a game, so this session was disconnected. It is the in-game version of error 264, and it can also appear if a warning or a ban lands on the account mid-session.
If 273 keeps kicking you mid-game and no other device is in use, sign out everywhere, change the password, and watch for a warning or ban notice when you sign back in. Repeated 273 kicks with no other session and no warning point to a stuck session, which usually clears after a few minutes, or to an account issue that Roblox support needs to look at.
What is the difference between error 264 and error 273? Error 264 usually appears when joining, because the account is already active elsewhere. Error 273 kicks you out mid-game, because the account launched the experience on another device while you were playing, or a warning or ban arrived during the session.
Does error 273 mean I was banned? Not by itself. It most often means another device launched the game. If a ban or warning was the cause, Roblox shows that notice when you sign back in or visit the account page.
Why does 273 happen when I only own one device? A session can stay open in the background after a crash, or the account may be signed in somewhere you forgot, such as a phone or console. Signing out of all other sessions ends those sessions.