problem reinstalling Snow Leopard

Hi.
I've bought a new Macbook Pro, and need to prepare my old 2011 Macbook for resale.
So far I have:
1. Deauthorised iTunes
2. Erased the drive
Now when I try to reinstall Snow Leopard from the DVD, I get to the software licence agreement, agree that, then a screen appears headed 'Install Mac OS X'. But underneath is a picture of a drive with a yellow warning triangle, and below that the words 'Mac OS X cannot start up from this disk'.
And that's as far as I can get.
Could someone please advise what I can do (in simple language please!)
Many thanks

What OS version was installed on your MBP before deciding to sell it?

Instead of erasing the drive...you may need to reformat it.

- Nick

Most 2011 models came with Lion hence Snow leopard will not install. As a rule of thumb no going backwards with operating systems.

For what it's worth. "Early 2011" MBP's originally came with 10.6.x (I have two)...and "Late 2011" MBP's came with OS 10.7.x.

This was one of the positives of an Early 2011 MBP (to me when I bought them). I could run 10.6 if I REALLY needed to...just in case I still had some PPC apps...and needed to run them via Rosetta.

- Nick