Can I install Xcode on El Capitan

You may need to get a "Free" Apple Developer's Account (ADC) and then get the El Capitan Xcode version from

https://developer.apple.com/download/more/

Anyone can get a free ADC account at:

<https://developer.apple.com/register/index.action>

As Bob has pointed out, you will need a free developer account, and you can use the same Apple ID and password to enroll as you use to enter this support community. If I recall, it is Xcode v8.3 that was the last version for installation on El Capitan. There have been big changes to Apple frameworks and Xcode itself in the subsequent two major releases of it.

Xcode 8.2.1 is the newest version to work on El Capitan