The NppJSONViewer plugin works just fine with v8.7

Please share your ?-menu’s Debug Info

But my initial guess, based on the error message, is that it looks like either you have the 32-bit Notepad++ and installed the 64-bit plugin, or you have the 64-bit Notepad++ and installed the 32-bit plugin. (I always forget which direction the phrasing of that error message goes). Did you use the Plugins Admin to install the plugin, or did you do it manually (if you did it manually, my guess is more likely to be right)

update: plugin v2.0.7 is the one available through the Plugins Admin… so you probably grabbed the wrong version when manually updating to / installing plugin v2.0.8.

update 2: I confirmed that it’s the error message that you get when you put in the 32-bit DLL or ARM64 DLL when you are running the 64-bit Notepad++. I just verified using NPP v8.7 and NppJSONViewer plugin v2.1.0 – you can download the most recent version here: make sure that if your Debug Info shows Notepad++ v8.7 (64-bit) that you need to grab the right download of the plugin:

(since there was a newer version of the plugin than you grabbed, I recommend using that version rather than trying again with v2.0.8)