You may receive the message 'IM-0261 [Screen Name] is unavailable' because you are trying to initiate a buddy chat, video chat, voice chat, or file transfer with a buddy who is not currently accepting messages from you. Ask your buddy to unblock you.

Unblock a Screen Name
Unblocking a screen name is fairly easy.

To unblock a screen name in AIM 6.1:
1. On the AIM Buddy List window, click the Edit menu, then click Settings....
2. Click the Privacy tab.
3. Under Block List:, click the desired screen name, to highlight it, then click the Remove link.
4. Click the General IM tab.
5. Under Buddy Chat, click the Block incoming Buddy Chat invitations box to clear it.
6. Click the Save button.