Any mac that came preinstalled with an OS newer than Mac OS X El Capitan will refuse to convert the InstallMacOSX.pkg to Install OS X El Capitan.app. You need to have a mac that can run El Capitan to do that, therefore any mac that at one point in
its life that could have run El Capitan.
If you want to be able to use that mac you need to install El Capitan on an external hard drive.
Your repeated use of the phrase bootable USB is causing the confusion.
Normally a bootable USB is used to describe a USB stick that has been configured
so that it can be used to install a version of Mac OS X or macOS on a Mac.
Something that you could try as you already have a bootable USB installer, plug it in to the MacBook.
Connect an external hard drive to the MacBook. Start the mac up while pressing and holding down the option/ alt key.
Wait a couple of minutes until the Startup Manager shows, select the bootable USB stick, press Return.
Once booted to the USB stick select Disk Utility. Highlight the external HD Disk icon, not any indented Volumes.
click on Erase, give the Disk a name, Format: Mac OS X Extended (Journaled), Scheme: GUID Partition Map.
Click Erase. After that is done start the installation of El Capitan, make sure you select the external drive as the destination disk.