If your Nokia n91 displays a message saying that the sim card is not vailid or sim card registration failed, try the possible soultions below, should help fix it:
1)When this error happens, remove the battery, put the battery back in and switch on your mobile phone.
2)Replace your sim card because somtimes it can be the sim card that is the cause to this problem.
3)Reset your Nokia n91, using a reset code to restore it back to factory settings.
4)Update your n91 to the latest firmware version.
5)Clean the sim card connetions, with a soft damp cloth.