Cant create a VM with OSX 10.6

I need to keep some legacy software under 10.6 and would like to use Parallels to create a virtual machine to run 10.6.
When creating the VM it asks for the 10.6 Server disk. Witch I happen to have (Snow Leopard Server) but it keeps telling me that it cant install the software.
Also tried with regular Snow Leopard (10.6) with the same negative result.

Please advice,

Roger