NoValidHost: No hosts found

I can't run instance,

when execute "euca-run-instances ami-sfipmfmy --kernel ami-ko6e7utb --ramdisk ami-ujs7u9y0 -k mykey"

the status is always like this:

[root@localhost dowmload]# euca-describe-instances
RESERVATION r-bevirrog PRO3 default
INSTANCE i-00000002 ami-sfipmfmy scheduling mykey (PRO3, None) 0 m1.small 2011-03-01 05:08:37.620438 unknown zone

and in "nova-scheduler" occurs errors like this:

[root@localhost dowmload]# nova-scheduler
2011-03-01 13:03:43,121 AUDIT nova.root [-] Starting scheduler node (version 2011.1-workspace:tarmac-20110202111800-10sib5zxlu8xluca)
2011-03-01 13:08:39,242 ERROR nova.root [-] Exception during message handling
(nova.root): TRACE: Traceback (most recent call last):
(nova.root): TRACE: File "/usr/lib/python2.6/site-packages/nova-2011.1-py2.6.egg/nova/rpc.py", line 192, in receive
(nova.root): TRACE: rval = node_func(context=ctxt, **node_args)
(nova.root): TRACE: File "/usr/lib/python2.6/site-packages/nova-2011.1-py2.6.egg/nova/scheduler/manager.py", line 63, in _schedule
(nova.root): TRACE: host = self.driver.schedule(elevated, topic, *args, **kwargs)
(nova.root): TRACE: File "/usr/lib/python2.6/site-packages/nova-2011.1-py2.6.egg/nova/scheduler/chance.py", line 37, in schedule
(nova.root): TRACE: raise driver.NoValidHost(_("No hosts found"))
(nova.root): TRACE: NoValidHost: No hosts found
(nova.root): TRACE:

any ideas ???