The USERTrust root certificate you are looking for was added in Sierra, and was not present in El Capitan.
If you can upgrade to Sierra or High Sierra or Mojave, you'll have the USERTrust root certificate. There are other reasons to upgrade to High Sierra (or later), if your Mac supports that. (Have an external backup or two, check that your key apps will work, check that your scanners and printers have drivers available from the hardware vendors, and upgrade.)
Otherwise, you'll have to acquire and load the certificate manually.
This is all part of the Comodo vendor reorganization. The certificate-related folks formerly known as Comodo are now known as Sectigo, and I think this is the root certificate certificate that you'll need to load, if you can't upgrade to Sierra or High Sierra or Mojave. There are variations of this certificate, depending on how current the system is, and yours isn't very current. I also don't have any way to test this, as I don't have an El Capitan system around.
Some organizations using this USERTrust certificate have (purported) downloads of the root certificate available.
You probably shouldn't trust me or that certificate download, either.