How to update Late 2011 Macbook Pro past 10.7.5
In order to upgrade to macOS High Sierra, you should be running at least OS X Mountain Lion. I suggest you upgrade to macOS Sierra first, which you can get here: Get macOS Sierra.
You can check here for more information on upgrading to macOS Sierra: How to download macOS Sierra

You may need to reset your Mac's software update URL. You can follow the guide at Robin's Blog to learn more about this. This excerpt from the post explains how to reset the update URL;


  
  Open the /Library/Preferences folder on the main hard drive.
  Find the com.apple.SoftwareUpdate.plist file and move this to the desktop (the easiest way is just to drag and drop the file to the Desktop). If you find a similar file with a .apple at the end (com.apple.SoftwareUpdate.plist.apple) then move that too. Moving them to the desktop means that you can restore them if this goes wrong.