Did you try to 1) delete the whole config but the root password 2) apply your own preprovisioned VC configuration with this single switch defined as member ID 0 and routing-engine role (you will have to use P6893-C if this is true for that switch, as its serial-number, as the shorter SN that is printed on its label will not work) 3) commit and reboot 4) give it some minutes after reboot.
Once it has this applied you could further try to use 'request virtual-chassis reactivate' to make this one become master and active.