I am inserting alphabetic letters to the message on my Nokia device, but the letters don't display in the order of letter input.Why?

You have the Predictive text function on. 

To switch off the predictive text function, select Menu > Tools > Settings > GeneralPersonalisation > Language > Predictive text > Off.

Or in some devices (e.g. Nokia N97), select Menu > Settings > Phone > Language > Predictive text > Off.