The built sketch I uploaded successfully to ARDUINO NANO EVERY using ARDUINO IDE. Off course selection of platform and COM port must be correct. The upload report is stored in upper part of Attachment.

On the other hand, uploading the same sketch using Visual Micro fails – see bottom part of Attachment.

Maybe I'm wrong: In the case of VisualMicro, I feel the messages "Upload method will be: bootloader" and "Uploading via Bootloader" are suspicious. 
I guess this probably does not correspond to the fact that the ATmega 4809 does not contain a bootloader at all and is programmed by the UPDI pin via ATSAMD11 as an interface.

In Visual Micro I selected "Optional hardware programmer: No Programmer (Default)". Theoretically, it is possible to choose from two Optional HW programmers in VM: "Onboard ATMEL mEDBG (UNO WiFi Rev. 2)" and "AVR Dragon as ISP". None of them resemble the jtag2updi used in the Arduino IDE and thus uploading does not work with them.

In ARDUINO NANO EVERY -> boards.txt the jtag2updi protocol is specified:
nona4809.upload.tool=avrdude
nona4809.upload.tool.default=avrdude
nona4809.upload.tool.network=arduino_ota
nona4809.upload.protocol=jtag2updi

but protocol jtag2updi is clearly not applied.  I have no idea Whether and how I should add the list of programmers in VS/VM.

Used SW:
VisualMicro (V.2023.220.1905) is an extension of Visual Studio Community 2019 (V.16.11.24)
Arduino IDE V. 1.8.12
Arduino Platform: Arduino megaAVR Ver. 1.8.8 

How to solve the issue of uploading ARDUINO NANO EVERY from Visual Micro?

Hello, Tim,
I modified the environment according to your instructions in a separate email. The existence or non-existence of the Local-Board.txt file does not seem to matter. The behavior is the same in both cases. As a test, I added a nonsensical 'asdf' parameter. This too had no response.
The behavior changed when I set Compatibility>Allow Local Board.txt = true. See protocol below. But even in this case, the (non)existence or content of Local-Board.txt did not matter.
Sincerely
Jirkaptr

The "vMicro>Uploader>verbose" parameter was set to TRUE in previous examples - see the attachments of both reports. It's set TRUE even now.

I manipulated the Local file Board.txt according to the instructions. Also I returned the "Allow Local Board.txt" parameter to FALSE.

Unfortunately, I did not understand your other instructions.

New knowledge:
1. A correctly uploaded Arduino NANO Every sketch communicates correctly with the Arduino IDE monitor and with two other terminals via COM3.
2. Another project with an Arduino NANO board (the old one!) uploads and communicates with Visual Micro without a problem.
=>   So maybe the problem is not in the upload, but in the communication?

May be I found the cause of the uploading and monitoring problem when using Visual Micro.

It is important to set the "Communication>Friendly Port Lists" parameter to false. In this case, a minimal text string e.g. 'COM3' indicating the port is well acceptable for the VM uploader and for avrdude (parameter -P).

If set to true, then the text string 'COM3 - Arduino NANO Every' is probably too long (27 character string) or incomprehensible for the VM uploader and possibly for avrdude (parameter -P).

In Windows Device manager, the port is listed as "Arduino NANO Every (COM3)".

I hereby draw your attention to a possible discrepancy between the features of VM resp. avrdude and very long name of the microcontroller port declaration.

I consider the case resolved for myself.

I attach a report of erroneous behavior.