Hi, Nasir! You're running out of time to apply to Google Summer of Code! Please see Where to start. I won't be watching this talkpage for response -- if you need help, please ask the chat channel (IRC) or the mailing list.Sharihareswara (WMF) (talk) 16:43, 26 April 2013 (UTC)
Look in the New signature: text box for your custom signature code. This contains the name of an account, but it's not your account. It's probably the name of another account that you used.
Change the wrong account name to your current account name.
If you have more than one account, and you are trying to link to the other account, then change at least one of the links to your current account name. For example, change [[User:Username|Username]] ([[User talk:Username|talk]]) to:
[[User:Username2|Username2]] ([[User talk:Username|talk]]) (one link to the current account's userpage, and one link to your other account's talk page), or to
[[User:Username|Username]] ([[User talk:Username|talk]] • [[Special:Contributions/Username2|contributions]]) (two links to your other account and one link to [[Special:Contributions]] for your current account).
Click the blue "Save" button at the bottom of the page. (The red "Restore all default settings (in all sections)" button will reset all of your preference settings, not just the signature.)
After you have done this, the error will be fixed. If the error is not fixed, then clicking the blue "Save" button at the bottom of the page will produce an error.