Yes if your installer pre dates October 2019 it will now be refused as its certification ran out in October last year.
As mentioned reset the date and time to a date prior to October 2019 and the installation should now go ahead.
If the mac is running with a working OS you can simply do this from System Preferences> Date & Time.
If however you do not have a working OS you can reset the date and time from Terminal after you have booted up to your USB installer.
Boot to your Recovery HD, click on Utilities in the menubar select Terminal.
Make sure WiFi is switched off, it can reset the date back to today.
Enter a new date, for example or just copy and paste
sudo date -u 011421002016
press Return
enter your password
press Return
If Terminal returns an error saying sudo : command not found, then try again without sudo.
Now try downloading the OS.
If this works then when the OS is installed and booted up you can Open System Preferences> Date & Time
and reset the time back to today.
Your 2011 iMac can be upgraded further if you wish, it is possible to upgrade to El Capitan or the newer High Sierra.
How to upgrade to OS X El Capitan – Apple Support
How to upgrade to macOS High Sierra - Apple Support
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOSHigh Sierra.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
