Error code 264 means Roblox sees the same account active on another device, so it kicked this session to stop two sessions running at once. It usually means the account is signed in on a phone, tablet, console, or another computer, or that someone else is using the account.
If 264 keeps appearing when nobody else is using the account, sign out everywhere, change the password, and check the account's recent login activity in settings. A session that keeps coming back from an unknown device points to a compromised login, not a Roblox bug.
Is error code 264 a ban? No. It is a session conflict, not a ban or a warning. The account is fine, Roblox just refuses to run the same account in two places at once.
Why does 264 keep happening when I only use one device? A previous session may still be running in the background, or the account may be signed in on a device you forgot about, such as a phone or console. Signing out of all other sessions clears it.
Can someone else trigger 264 on my account? Yes, if they have the login and launch Roblox on their device. If that happens and you did not share the account, change the password and turn on two-step verification.