If you simply want El Capitan running again (not an ISO file), do this:
Once you have the official InstallMacOSX.dmg or the Install OS X El Capitan.app: Mac Os X 10.11 El Capitan Iso Download
# Create a blank CDR image
hdiutil create -o ElCapitan -size 8g -layout SPUD -fs JHFS+
Before we discuss the ISO download, let’s address compatibility. You should only install El Capitan if: If you simply want El Capitan running again
Important Note: Apple no longer actively supports El Capitan. The last security update was released in 2018. Do not use this as your daily driver for banking or sensitive data. Important Note: Apple no longer actively supports El
Apple never officially distributed El Capitan as an ISO. The official installer came as a .app bundle inside a disk image (.dmg) or downloaded directly from the Mac App Store. ISOs you find on third-party sites are often homemade conversions — sometimes fine, sometimes risky.
Parallels can directly use the .app installer; you don’t need an ISO. Just drag the Install OS X El Capitan.app into Parallels.