You can have a look here Don't have your Server Admin Query password? Look here!
There you’ll find all different methods to override the default password for the serveradmin Query password.
After the server is restarted and the new password is applied, you should be able to create a Snapshot like this:
1 is in this example your VirtualServer ID
serveradmin> use 1
error id=0 msg=ok
serveradmin@9987(1):online> serversnapshotcreate
version=3 data=KLUv\/WT81iWYAAqJQBpFEGto25gCAAAAQF