Trying to install old versions of Mac OS (including El Capitan) after support for them has expired can sometimes result in errors like "no eligible packages" being available. This is usually because the date you are doing the install on is after the certificates have expired. Try resetting the computer's date back to a time that the install was valid, like this...
1 Boot from the installer. 2 Go to "Utilities" menu, choose "Disk Utility". 3 Erase the disk as "Mac OS Extended (Journaled)". 4 Quit from Disk Utility. 5 Run Terminal and set the date to mid 2017 (date 060101002017). 6 Shutdown and reset the SMC. 7 Boot from the installer again. 8 Try to install El Capitan.
TIPS PAGE