Hi,
This is because /dev/da0s2a cannot be mounted.
error: cannot mount /dev/da0s2a
You could go to root shell and check if the partition is already mounted. If yes, you may need to unmount the partition and then try snapshot slice alternate again:
start shell mount -l umount /dev/da0s2a
cli
>request system snapshot media internal slice alternate
You may need to force the unmount using 'umount -f'
Or you can follow:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28238&actp=search
Just to be cautious to do this in a MW.
Cheers,
Ashvin