Anti-virus software works by searching any incoming data for known viruses. It has a database of all known viruses which, of course, needs regular updating because people keep creating new ones. Updating the virus database is normally free and usually automatic.
As a separate entity, the software to search for viruses is licensed and can only be used for eg, a year.
It sounds like your software licence is still valid, but the database needs updating; perhaps yours doesn't do automatic updates or it has missed one for some reason.