Hello:
I'm trying to call the pop-up palette with my tablet Huion H420 using the pen's second bottom. I've settled it to perform the mouse's right buttom but Krita isn't calling the palette when I press it.
I'm using Windows 8.1

Has this problem only just started after some time of using krita and your Huion H420 without any problems? Or, have you only just started using the Huion H420 and/or krita. Some history of the problem development would be useful to know. Which version of krita are you using?

I ask because any version of krita running on my Windows 10 system now has strange problems with my Wacom stylus and a similar inability to recognise a stylus button click. I suspect a Windows update has done this since everything was fine about two weeks ago and I haven't changed anything. Other graphics type applications (such as GIMP) are showing the same problem.

Please check if you have WinTab Tablet API set in Preferences->Tablet dialog (needs restart after changing)