hi, i try to launch:

openstack server create --flavor y --image y --nic net-id=external --security-group default --key-name y y9
openstack server show y9
| OS-EXT-SRV-ATTR:instance_name        | instance-000002fe                               
| fault                                | {u'message': u'No valid host was found. There are not enough hosts available.', u'code': 500,

where

openstack flavor show y
+----------------------------+--------------------------------------+
| Field                      | Value                                |
+----------------------------+--------------------------------------+
| OS-FLV-DISABLED:disabled   | False                                |
| OS-FLV-EXT-DATA:ephemeral  | 0                                    |
| access_project_ids         | None                                 |
| disk                       | 10                                   |
| id                         | bbacb163-d9b8-4efe-acd9-c18334f7941f |
| name                       | y                                    |
| os-flavor-access:is_public | True                                 |
| properties                 |                                      |
| ram                        | 512                                  |
| rxtx_factor                | 1.0                                  |
| swap                       |                                      |
| vcpus                      | 2                                    |
+----------------------------+------------------------------------
openstack image show y
+------------------+--------------------------------------+
| Field            | Value                                |
+------------------+--------------------------------------+
| checksum         | 5dd393eca8de6e61d1cdb8b7ec3852e8     |
| container_format | bare                                 |
| created_at       | 2019-01-22T07:58:16.000000           |
| deleted          | False                                |
| deleted_at       | None                                 |
| disk_format      | qcow2                                |
| id               | a6f914aa-eefe-477f-b9ee-ef05953c3f3c |
| is_public        | False                                |
| min_disk         | 0                                    |
| min_ram          | 0                                    |
| name             | y                                    |
| owner            | 31b1b76a0163418f9d5556836f44f67d     |
| properties       |                                      |
| protected        | False                                |
| size             | 2747006976                           |
| status           | active                               |
| updated_at       | 2019-01-22T07:58:36.000000           |
| virtual_size     | None                                 |
+------------------+-----------------------------------
openstack keypair show y
+-------------+-------------------------------------------------+
| Field       | Value                                           |
+-------------+-------------------------------------------------+
| created_at  | 2019-01-21T08:05:30.000000                      |
| deleted     | False                                           |
| deleted_at  | None                                            |
| fingerprint | 54:0f:2b:1f:1d:8a:1f:24:49:07:ef:8a:77:80:2d:15 |
| id          | 2                                               |
| name        | y                                               |
| updated_at  | None                                            |
| user_id     | 3f93876ec2104c828bb47110b6fecf82                |
+-------------+-------------------------------------------

i changed in /etc/nova/nova.conf

openstack-config --set /etc/nova/nova.conf DEFAULT compute_driver libvirt.LibvirtDriver
openstack-config --set /etc/nova/nova.conf libvirt virt_type kvm

then i tried again ->the same error then changed kvm with qemu ->the same.