pvesm status
Depending on what storage technology you use you may expand the logical volume, or increase a zfs volume size as long as you have usable disk space in general.
Code:pvesm status
If your harddrive's are full then I'd add additional storage? Or (some) move existing backups offsite/to an external storage.
What kind of storage do you have?
@t.lamprecht gave advice, without knowing what kind of storage you have available it is not possible to give more advice.
By the way, please post code and command outputs in CODE tags.