Your Mac is JUST on the cusp of what is technically able to upgrade to El Capitan. Requirements:
MacBook (Late 2008 Aluminum, or Early 2009 or newer)
OS X v10.6.8 or later
2GB of memory
8.8GB of available storage
If you have enough storage then there might be an issue with a corrupted installer. For the most helpful troubleshooting I really do recommend getting in touch with Apple Support.