In retention disable the option "Remove index/logs even if backup file cannot be deleted".
If this option is enabled NovaBACKUP (NBK) will remove an index even if it is not able to delete a file on the disk. At next execution the retention will NOT care about the files! If you use changeable media (RDX or ext. HD) these media will run out of free space then. Same is valid for NAS systems that may be offline for a while.
In these cases you have to delete the files manually.
The retention needs full access rights to a storage destination (share). In the schedule add a user with granted full access rights to source and destination. Possibly you have to modify the service "NovaStor NovaBACKUP Backup/Copy Engine" from running with "local system account" to an admin account.