My Experience

Insert SIM card problem may happen and shows on the LCD Lcd display, if the SIM card inserted on the mobile phone device is already damaged but IF the SIM card card is working good and the device handset will not accept any kind of SIM card, the problem may lies on the device itself. This may happen if the mobile phone is damaged by a liquid or water damaged and any means of accidental damage by the standard user.

Heres solution that may help possibly help fix and repair Nokia X6 Insert SIM card problem. 

 This solution shows the SIM card circuit on Nokia X6, including The line tracks of the SIM card pin connector through to each corresponding solder ball bumps of the IC components (AVILMAS) that holds the SIM card to work.