Mac Book Pro won't download El Capitan

reboot into safe mode

Try safe mode if your Mac doesn't finish starting up - Apple Support

then try the download again

if it fails quit out of the app store

open the Terminal (type "terminal" w/o the quotes in the spotlight)

and type

open $TMPDIR../C/

press return

delte the folder

com.apple.appstore

quit terminal

open app store again

it is HIGHLY ADVISED before performing this update you backup your computer before you update.

it is also advised any device connected to your computer and any 3rd party software is verified to work with this update by inquiring with the developers of the site.

While sites like "roaring apps" can provide some information on this these sites are often not regulated and can be full of erroneous information and inaccuracies.

Hi Richard, I assume you are trying to download El Capitan from the Mac App Store. If that's the case, I suggest you wait a while and try again. There are often problems with MAS due to bandwidth, If you are using a different site then it is probably an El Capitan update.

EDIT:Also Jimmy has some great suggestions.

[edit] the last paragraph I edited before Mac suggested my suggestions were any good but there are plenty of users here who have updated only to find their printer or some other software is suddenly borked and a backup is worth it's weight in gold.

Use Time Machine to back up or restore your Mac - Apple Support