"plist parsing error" probably means "property list parsing error". I believe that macOS uses structured ".plist" (property list) files to store a lot of preference settings. I would not be surprised to find that the communication between the App Store application (on your Mac), and Apple's servers, makes use of similar structured data.
That suggests we might be looking at an issue where the version of Mac OS X or Recovery mode that you are now running was not able to make heads or tails out of some piece of data it received from the App Store. Maybe because that 2011 MacBook Pro is so old that the App Store might have changed in some fashion that makes it incompatible with the recovery mode you are in now?
If you had a working version of Mac OS X, you could try using a modern Mac or PC to download the .DMG with the Sierra installer, so you could transfer that .DMG to the old Mac and run the installer inside. Once you were running Sierra, you could try using the old Mac itself to access the High Sierra App Store link.
How to download and install macOS - Apple Support
Unfortunately, if you have nothing to boot from (erased the internal drive and have no bootable backups), you may be up the creek without a paddle …