I Mac 24 inch will not let me reinstall El Captain on a erease drive

That's really informative. What is exactly happening. Are you trying to install El Capitan through the Recovery servers

and receiving an error message. Or are you trying to install El Capitan from a bootable USB or by some other means.

It would really help a lot if you give a detailed description of your problem.

First, you need to install Snow Leopard before you can upgrade to El Capitan.

Upgrading to El Capitan

To upgrade to El Capitan you must have Snow Leopard 10.6.8 or Lion installed. Download El Capitan from the App Store. Sign in using your Apple ID. El Capitan is free. The file is quite large, over 5 GBs, so allow some time to download. It would be preferable to use Ethernet because it is nearly four times faster than wireless.

      Macs that can be upgraded to OS X El Capitan

         1. iMac (Mid 2007 or newer) - Model Identifier 7,1 or later

         2. MacBook (Late 2008 Aluminum, or Early 2009 or newer) - Model Identifier 5,1 or later

         3. MacBook Pro (Mid/Late 2007 or newer) - Model Identifier 3,1 or later

         4. MacBook Air (Late 2008 or newer) - Model Identifier 2,1 or later

         5. Mac mini (Early 2009 or newer) - Model Identifier 3,1 or later

         6. Mac Pro (Early 2008 or newer) - Model Identifier 3,1 or later

         7. Xserve (Early 2009) - Model Identifier 3,1 or later

To find the model identifier open System Profiler in the Utilities' folder. It's displayed in the panel on the right.

You can purchase Snow Leopard through the Apple Store: Mac OS X 10.6 Snow Leopard - Apple Store (U.S.). The price is $19.99 plus tax. You will be sent physical media by mail after placing your order.

After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store. Access to the App Store enables you to download El Capitan.

Snow Leopard Erase and Install

1. Insert Snow Leopard DVD into the optical drive and restart the computer.
2. Immediately upon hearing the chime hold down the C key.
3. Release the key when the Apple logo appears and wait for the loading to finish.
4. After the installer loads select your language and click on the Continue  button. When the menu bar appears select Disk Utility from the Utilities menu.  After Disk Utility loads select the out-dented disk entry from the side list (mfgr.'s ID and drive size.)  Click on the Partition tab in the Disk Utility’s right window.  Set the number of  partitions to (1) from the drop down menu. Click on Options button and select GUID, click on OK, set the format type to MacOS Extended, Journaled. Click on the Security Options button and set it for one-pass then click on the OK button. Finally, click on the Apply button.
5.  After formatting has finished quit Disk Utility.  Continue with the OS X installation and follow the directions.
6.  When the installation has finished the computer will restart into the Setup Assistant. After you complete the Setup Assistant you will be running a fresh installation of Snow Leopard 10.6.3. Open Software Update and install the recommended updates.

Download and install Mac OS X 10.6.8 Update Combo v1.1.