This monitor checks the existence and age of database backup as reported by Microsoft® SQL Server™. This is performed by running a query against the master database of the SQL instance and returning the age of the database backup. Note that the monitor ignores Always On databases. Therefore, it is always "green" for those databases. For backups of Always On databases, use the dedicated monitors at the Availability Group.
Database has not been backed up for too long.
You should perform a database backup to resolve the issue.
To eliminate this issue in future it is recommended to schedule an automated backup according to the needs of the application and business requirements.
Best Practice recommendations for SQL Server Database Backups