The OS cannot be installed or booted for the unsupported version.

I try always to have a recent bootable backup on hand anyway, so that if my machine goes south I can be up and running again with minimum downtime. A list of compatible macOS (OS X) versions is posted for each Mac model. (In fact, my backup strategy includes a 1TB USB drive, with two encrypted partitions, one for Time Machine and one for a full clone.) Then you can revert to Mavericks if you need. Battery has a cycle count of 704 and comes with a charger.

Unless you're really squeezed for cash, I would suggest getting a USB hard drive for $50 and using Carbon Copy Cloner or SuperDuper to make a full bootable backup of your Macbook before upgrading. MacBook Pro 15' Core 2 Duo 2.66 GHz El Capitan 320 GB HD - 8 GB RAM in very good condition. El Cap feels a lot faster on both MacBooks than it does on either of the machines with hard drives. My impression is that the real limiting factor in performance isn't RAM it's whether you have an SSD or a spinning hard drive. I recently bit the bullet and upgraded from Snow Leopard to El Cap on my MacBook Pro with Retina (15', mid-2012, 8 GB), my MacBook Air (11', mid-2013, 4 GB), my Mac Mini Server (4 GB), and my work iMac (8 GB).

El capitan on 2006 macbook pro