Give it some time for the image to download and create first.  When ready, instantiate again:

[oneadmin@opennebula01 1]$ ls -altri
total 647896
 68474907 drwxrwxr-x 5 oneadmin oneadmin        30 Mar 30 18:58 ..
101870759 -rw-r–r– 1 oneadmin oneadmin 395074048 Mar 31 02:41 2a3d337ded54dcbcc35865daf54e3c9a
101870807 drwxrwxr-x 2 oneadmin oneadmin        84 Mar 31 02:41 .
101870760 -rw-r–r– 1 oneadmin oneadmin 174383104 Mar 31 02:42 bf2a2c5916e78efcd0223b9c98971278
[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$ onevm list
    ID USER     GROUP    NAME            STAT UCPU    UMEM HOST             TIME

[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$ onetemplate instantiate "CentOS-7"
[TemplateInstantiate] Error allocating a new virtual machine. DISK 0: Cannot acquire image 3, it is in state: LOCKED
[oneadmin@opennebula01 1]$ ls -altri
total 771632
 68474907 drwxrwxr-x 5 oneadmin oneadmin        30 Mar 30 18:58 ..
101870759 -rw-r–r– 1 oneadmin oneadmin 395074048 Mar 31 02:41 2a3d337ded54dcbcc35865daf54e3c9a
101870807 drwxrwxr-x 2 oneadmin oneadmin        84 Mar 31 02:41 .
101870760 -rw-r–r– 1 oneadmin oneadmin 395074048 Mar 31 02:43 bf2a2c5916e78efcd0223b9c98971278
[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$
[oneadmin@opennebula01 1]$ onetemplate instantiate "CentOS-7"
VM ID: 24
[oneadmin@opennebula01 1]$ onevm list
    ID USER     GROUP    NAME            STAT UCPU    UMEM HOST             TIME
    24 oneadmin oneadmin CentOS-7-24     pend    0      0K              0d 00h00
[oneadmin@opennebula01 1]$