Trying to reinstall El Capitan but installation failed

What error are you receiving when attempting to install El Capitan?

Do you have the 13" non-Retina model? If so you could have a defective internal hard drive cable which is common with that specific model. Or you may have a failing hard drive. If your laptop wasn't working correctly before the attempted reinstall of macOS then this could definitely be the source of the problem.

I would recommend purchasing an SSD and replacing one of the original 2GB memory modules with an 8GB module to give you 10GB of memory. Also replace the internal hard drive SATA cable especially if you have the 13" model. Crucial and OWC are great sources for an SSD and memory since Macs are really picky about the memory they use. If you go with a Crucial SSD go for the MX500 and stay away from the BX500 which can be as slow as a hard drive.

Make sure to have good verified working backups. This is even more important with SSDs where it is impossible to recover accidentally deleted data plus SSDs can fail at any time without any warning signs.

Good luck.

Try internet recovery:

on boot press and hold command+option+r and you’ll see a spinning globe. After that connect to the internet and it’ll download OS X for you. Then try reinstalling OS X again.