Seems that by searching the web, a reason for this message has been located;
and is a symptom of using an older out-dated installer download. This can still
be used by changing the date of the install. System clock must be set back
to before the issue/time or outdated status to make use of this older installer.
You could also try and create a bootable installer from newer (or El Capitan) installer.
• How to create a bootable installer for macOS - Apple Support
https://support.apple.com/en-us/HT201372
Same topic is covered with similar answers (to use older installer, if not getting
newer one from Apple server).. From duckduckgo results, without opening any
resulting pages; note the similarity:
https://duckduckgo.com/?t=ffab&q=copy+of+the+install+os+x+el+capitan+application+can%27t+be+verified&ia=web
One that shows (in my browser, right side) has similar 'instruction detail' as others.
"This copy of the Install OS X El Capitan application can't be verified.
It may have been corrupted or tampered with during downloading"
http://apple.stackexchange.com/questions/216730/ddg#219067
To change the date/time to make this work, you may/need use Terminal.
In above suggestion; and how to enter date Correctly, is also shown.
Not sure what else to suggest; if your macOS Installer has been in the
Applications folder awhile, newer one had been created by Apple and
available as download from their Server via URL. Move old one to Trash
and Empty. Download newer one ~
It should work and with that, no need to use Terminal would exist.
The newest installer (for El Capitan) would be found here:
• How to upgrade to OS X El Capitan - Apple Support
https://support.apple.com/en-us/HT206886
Instructions to use the installer are the point of article.
The part that is the Installer (not the OS, only installer)
separately obtainable here:
"..If you still need OS X El Capitan, use this link: Download OS X El Capitan.
A file named InstallMacOSX.dmg will download to your Mac.."
File hidden in the blue URL link has newer date than file that caused your issue:
http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg
If you've done all the above (except change date with Terminal) try & do that.
Good luck & happy trails!🌞🎣