1. You have created a backup of a virtual machine with large virtual disks (disk size more than 300Gb),
2. When you mount a backup using Run as VM option, operation finishes with the following error:
Failed to prepare operations. Error code: 7 'Disk is not found. Ensure that all the hard disks are installed in the machine.'
Issue in the product. The issue will be fixed in the next product builds.
As workaround, you can recover the backup to a new virtual machine in backup console or using Acronis bootable media.
How to recover a VM:
To recover a VM using bootable media, first create a new blank VM with the same parameters as the VM that you want to restore, boot the VM from Acronis bootable media and perform the recovery: