Is your USB installer for El Capitan an old one made before October 2019.
If so it will probably have invalid certificates and that is why you are seeing
that error message.
In October 2019 the certificates for several mac OS X's and macOS's expired.
Apple released newly certificated versions f these OS's and you can download
the revised El Capitan installer from here,
How to upgrade to OS X El Capitan – Apple Support
Go to Section 4 and click on Get OS X El Capitan.
This will download InstallMacOSX.dmg to your Downloads folder.
The next section can only be done on a mac that is capable of running El Capitan,
a mac that came preinstalled with an OS later than El Capitan will refuse to do the next bit.
When downloaded open to InstallMacOSX.pkg, double-click on
that and an installation window will open, this does not install El Capitan
but converts the InstallMacOSX.pkg to the Install OS X El Capitan.app which
you will find in your Applications folder, it should be 6.2GBs in size.
(If the installation window asks which disk you want to install to, you must pick
the disk that you are booted to at the time. Not any internal or external disk that
you want to eventually install El Capitan on, that is for later.)
It is also possible to create a bootable USB installer disk using the Install OS X El Capitan.app
in the Applications folder and the createinstallmedia command in the Terminal app.
Read the instructions here,
How to create a bootable installer for macOS – Apple Support
Once you have created the bootable USB installer restart your mac while pressing and holding
the option/ alt key. In a couple of minutes you will see the Startup Manager select the USB
and press Return.
The mac will now boot to the USB, you will see a Utilities panel, select Disk Utility press Continue.
Highlight the Disk not the indented Volume and click Erase.
Give the Disk a name,
Format: Mac OS Extended (Journaled)
Scheme: GUID Partition Map
Click Erase.
Quit Disk Utility.
Click on Install OS X.
The installation process will start, follow the prompts.
