Os X Server 10.6 install as guest fails on Parallels 13/El capitan

I have to reinstall Os X Server 10.6 as guest on Parallels 13/El capitan but it fails with the attached message. I tried from a disk image created by Disk utility app and directly from the dvd. Please, what can I check to fix it?
Thanks

 

Hi, did you try this KB article?

 

Thanks a lot. It is now blocked with this screen

 

Ok, I solved it. Restarting and remounting the installer dvd, the installation is started.
Thanks a lot.