Downloaded InstallMacOSX.pkg gives an error of "This version of OSX 10.11 cannot be installed on this computer." Why?
Can't you just do a reinstall of macOS on the other Mac by reinstalling OS X El Capitan that came with a Mac. This link will explain: https://support.apple.com/en-us/HT204904.

Also, make sure that the disk is formatted HFS or HFS+ not APFS.

Good luck!