Q: Why can't I install El Capitan now that Sierra has been released?

Apple removes the previous versions from the Mac App Store once a new one has been released.

If you did not at any point download El Capitan then there's no way to get it now.

If you did download it at any point, it can be obtained from the Purchased section of the Mac App Store.

At this point, no major issues have been reported with Sierra though.

Try If your Mac is not Compatible with Sierra but you Have Not Upgraded to El Capitan.

Now, your Mac IS compatible with Sierra, but I would be curious to know whether or not the procedure still works.

poiuyt1 wrote:

 

but this is for my wife's computer and i don't want her to be the guinea pig if problems do pop up -- which they often do on a first version of a new release.

A small number of users are affected by problems after a first release; the vast majority are not.

What you can do is partition the hard drive, and then on a secondary, small partition, install Sierra and see how it works. Be sure to make a Time Machine backup before partitioning. If you are not sure how to partition, see: Disk Utility (El Capitan): Partition a physical disk.

I really don't want to upgrade to Sierra on an older machine and risk having a lot of problems on a first release,

Please read Upgrading to macOS "Sierra" without fear.