Q: Problems installing El Capitan

You could try & see if the Startup Manager can see the older OS X on the HDD:

How to choose a startup disk on your Mac - Apple Support

You may try & see if by starting up in OS X Recovery, you can use the OS X Utilities

and select a different startup volume (such as existing pre El Capitan OS X) from there.

OS X: About OS X Recovery - Apple Support

If there isn't an undamaged or existing original OS X version installed or available on

the computer hard drive, the OS X Recovery may choose to try & get your original

computer OS X. The older system should be there; but it may be damaged.

If you have a time machine backup for the system previously running in the Mac before

this failed upgrade attempt, you could use that to attempt to restore the former OS X.

Various good ideas and suggestions are covered here:

http://pondini.org/OSX/RecoveryHD.html

Usually you can attempt a startup and force the mac to look for any OS X installed by

use of keyboard command shortcuts. Startup shortcuts and others, are available.

Startup key combinations for Mac - Apple Support

Most other keyboard combinations for Mac:

Mac keyboard shortcuts - Apple Support

Hopefully these ideas may help. There are several possibilities so they may serve to

confuse; read round the issue/topic before attempting.

Good luck!

And what does the error message say?