Well is your system date correct or not?
Check the system date, especially the month and year, correct if required.
Double click the system clock and check if the date, month and year are correct, if not correct it.
Check the about avast and see the update expiry date hasn't passed.
Right click the avast icon and select About avast!, is it a date in the future?
If not you will need to renew your Pro license.