I tried reinstalling el capitan into my new disk but it said item unavailable, try again later. What do I do?

Are you booted into the Recovery Partition? Have you ever downloaded El Capitan using your Apple ID?

You can try to download it at the link below. If it won't let you, then your only options are to upgrade to Sierra/High Sierra or find an older computer where you can download it using your Apple ID.

El Capitan

You can make a bootable USB stick to install using these free programs which will do all the work for you.

Bootable USB Flash Drive – Diskmaker X Disk Creator oruse Apple’s method Create a bootable installer for OS

Try booting into Internet Recovery (command - option/alt - R on a restart). It is a slow process. This will install the newest OS that the computer is compatible with. Reformat the drive using Disk Utility/Erase Mac OS Extended (Journaled), then click the Option button and select GUID. Quit Disk Utility. Then install the OS.When the computer restarts, you can use Setup Assistant to restore your data from Time Machine.

OS X Recovery

OS X Recovery (2)

Have you previously downloaded El Capitan using your Apple ID? What normally causes that error message is you haven't downloaded it using your Apple ID.

You are welcome. Glad to help.