Unable to update my 2008 iMac to OS X El Capitan 10.11

That com.apple.installer.pagecontroller error -1 typically means the installer file is either corrupted, not properly signed, or is being run on an unsupported system or from the wrong OS version.

The 7,1 would be a Mid 2007 model, not a 2008 one. It should be able to run El Capitan, but as den.thed has already stated, it will not be able to update your iPod (or iPad/iPhone) to the newer operating system.

Ref:

• OS X El Capitan - Technical Specifications
• How to download and install macOS - Apple Support

What's not clear from your post is, whether or not, you were successful in upgrading this iMac to El Capitan ... but, even with El Capitan, your Mac's ability to access secure (think financial) websites will be extremely limited, or non-existent due to the newer security requirements that these websites have imposed. This is, most likely, why you are seeing this error message.

Ref: Requirements for trusted certificates in iOS 13 and macOS 10.15 - Apple Support

What operating system is the iMac running? You can check by clicking  > About This Mac.

If your iMac is running Version 10.6.8 or later, your iMac is compatible with upgrading directly to OS X El Capitan. If it runs 10.6 but shows a different number, use the Software Update app to download and install the Mac OS X Update Combined to 10.6.8. If it runs a later version, there should be no problems with upgrading.

(do this on your iMac)

Click this link and select the download for El Capitan 10.11.

How to download and install macOS - Apple Support

When the InstallMacOSX.dmg file has completed downloading, open it and select the .pkg file contained within. When it finishes installing the application, check your Launchpad or Command+Shift+A on the Finder, for the app "Install OS X El Capitan". Open the app and follow through with the instructions. It should install as normal.

I hate to be the bearer of bad news, but that iMac is too old to support a newer macOS versions or reach the server for iPod updates.

FWIW even a later 2012 or 2013 iMac running macOS 10.15 Catalina, would be hard pressed to reach the server for iPod updates.