I guess you're using a trial version .Buy the license to change into a full Version
OP Wrote:
i remove the sql 2008 r2 completely from my machine and try to reinstall it again but every time i open the sql management studio it shows the same message.
It happens because of
Windows_Registry[
^].Even if you reinstalled the product ,the windows registry still holds the info about the expirydate which is registered at the very first time on installation of the product