Hello
v$instance is a synonym created on the view sys.v_$instance. You can try to change the sctipr so it looks like this.

select
instance_name,
host_name,
startup_time,
status from sys.v_$instance

I tried this and it works. I am not sure why you can't run the script as you have wrote it connecting as system/manager because that owner should have select privileges on any table or view.