New HP Envy Pro 6452 installation on linux

[vineyridge]

LInux Mint 19.3, 2009 ASUS BIOS computer

I am trying to install this printer and  installed HPLIP 3.20.5 on June 6th. I could not figure how how to make the printer work, since there was no manual available.  I couldn't even find the paper tray.  Somehow, I screwed up the printer and the printer controls.   Since then I have installed Linux driverless printing and scanning for USB, which works, but I still have a huge problem.  My control panel is gone unless it is giving me an error. 

I reinstalled HPLIP 3.20.5 today alongside the driverless printing/scanning and thought that might solve the control panel issue, but it hasn't.  The information provided for this printer says that HPLIP was installed on June 6th. 

I am getting this error message:

HPLIP Device Status

ENVY_ Pro_ 6400_ series Printer (serial number))

Device is busy, powered down, or unplugged. (5002)

The device is plugged in and powered on.  It may still be busy with multiple test page printing requests from June 6th, but the print queue shows as empty.  It prints, since I just printed a test page through the Mint Printer setup tool.  The Driverless printing printer still works also.

My recollection is that the control panel disappeared when I installed the Driverless printer and driverless Simple-Scan.

So I need help in fixing my printer from Linux knowledgeable people.

1.  How do I go about restoring the control panel?  Do I need to uninstall the driverless printer and scanner to return control to HPLIP,  so my printer is not seen as busy?

2. Scanning function:  From the HP Device manager on my monitor,  I get this message:

"Failed to open hpaio:/ENVY_Pro_series_6400_series (serial number)

Error during device I/O."

Then a window pops up telling that I am missing an HP Plug-In and offering to install it.  When I select "install", I get this error message:  " Failed to install Plug-in.  Either you have chosen to skip the Plug-In installation or entered an incorrect password."

I have not chosen to skip the Plug-In, and I have no idea what password it is talking about.  I then tried to install it through the terminal with the command "hp-plugin".  This didn't work either: "Downloading plug-in from:
Downloading plug-in: [\ ] 0% Receiving digital keys: /usr/bin/gpg --homedir /home/ajh/.hplip/.gnupg --no-permission-warning --keyserver pgp.mit.edu --recv-keys 0x4ABA2F66DBD5A95894910E0673D770CDA59047B9
()
error: Unable to recieve key from keyserver"

I was also warned that the plug-in should not be installed to root, but no alternate path was suggested.

How do I get a fully functioning printer with a working control panel, scanner, ADF, information,  and printer management and maintenance tools?

Edited to add that I just installed a clean copy of Mint and installed the printer there.  I think that it is fully functional on that OS.  I did do a full product reset before that installation.  And now everything that I have tried has worked on the original OS.

But now I have another question: