Interesting. My guess is that you are trying to use either the PPD from the HP drivers that are not supported or that the PPD has become corrupted. I would recommend you do the following:
1. Go to System Preferences -> Print & Scan and delete the DesignJet 120nr printer that is installed.
2. Go to /Library/Printers/PPDs/Contents/Resources (that is the Library at the root of the hard drive). Delete any PPD for the DesignJet 120 you find.
3. Download a fresh copy of the OpenPrinting.org PPD with this link.
4. Open the PPD with a text editor and remove the line
*cupsFilter: "application/vnd.cups-pdf 0 foomatic-rip"
5. Save the file and place it in the folder /Library/Printers/PPDs/Contents/Resources
6. Go back to System Preferences -> Print & Scan and add the DesignJet 120nr printer. When you add the printer, choose the model that says "HP DesignJet 120 Foomatic/cdnj500 (recommended)".
Try finding the options for Paper Feed and then Roll Paper.
Let us know how things go.
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