Using Window 10 and IDE 1.8.15 have been getting Upload Error on the Nano Every for days.
In desperation tried Cloud Editor.
In the drop down to the right of the arrow pointing right choose Select Other Board and
Port.
The Nano Every is selected. (Which may have been issue with desktop IDE because the Every did not seem to be available.) The dialogue box also says 'Select Other Board & Port Select both a BOARD and a PORT if you want to upload a sketch. If you only select a BOARD you will be able just to compile but not to upload your sketch.'
That was one of the issues. There is a dedicated topic for discussion of the problems @AllenPitts is having on the desktop IDE here:
Do you have Arduino Create Agent running?
If you open Windows Device Manager and then plug in your Nano Every, do you see a new port appear under the "Ports (COM & LPT)" section of the device tree?