Please investigate the datastore size and relative free space and adjust the notifications setting as needed. As the notifications are based on the percentage of free space and not a specific value, some environments with very large datastores may trigger these notifications sooner than expected. For example, with the default settings (warn=10%/skip=5%) in an environment with a 50TB datastore, the warning about free space will trigger when there is still 5TB of free space and begin skipping VMs at 2.5TB.
However, if the Datastore is actually low on free space, it is advisable to start by investigating whether any virtual machines using that datastore are running on a snapshot either user-created or one accidentally left behind by Veeam Backup & Replication. The snapshots created by Veeam Backup & Replication for vSphere environments will be named 'VEEAM BACKUP TEMPORARY SNAPSHOT', for Hyper-V environments, they are named 'Veeam Recovery Snapshot'. These snapshots should only be present on a VM when a backup/replication job is actively processing that VM.
Note: If the datastore is used to store replicas, remember that snapshots are used as restore points on replicas and should not be manually removed.