Re: Is "Backup" role required if I only have two switches?

You are not in CLI or edit mode:

root@test:RE:0% cli
{master:0}
root@test> edit 
Entering configuration mode

{master:0}[edit]
root@test# set virtual-chassis preprovisioned 

{master:0}[edit]
root@test# set virtual-chassis member 1 serial-number BM0211335826 role line-card 

{master:0}[edit]
root@test# commit