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
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.'