Microsoft SQL Server Express has a configuration data storage limit of 10 GB. The Express Edition is sufficient for the evaluation and small environments (500 VMs).
You can choose Microsoft SQL as a Veeam Backup & Replication database. You were installing a Microsoft SQL Server (or Express), either locally on the backup manager server or remotely. If Microsoft SQL Server is not already installed. In that case, the Veeam Backup & Replication won’t install the Microsoft SQL Server Express Edition on the backup server automatically. You must install it before installing Veeam Backup and Replication v12 version.