The only thing I can think of is, and this may sound stupid, but when you press the undo or redo key, do you have the stylus close enough to the tablet surface so that the clicks can be detected by the tablet?
And to help other helpers find the solution:
Your link to the photo unfortunately doesn’t work for me, could you please upload the image here on the forum?
Which Wacom Graphic tablet exactly do you use, and if there are different working pen models, which pen is it?
Which Windows version are you using?
So now I have to go to sleep - it’s 4:30 in the morning.
You wrote about the undo button on the pen. By default Wacom’s pen side buttons are mapped to middle and right mouse button. Did you have changed this to emit a ctrl + z and ctrl + shift + z signals instead?
From my experience you really have to set the pen to emit key strokes, setting it to Wacom’s “actions” wont work in Krita (at least it’s that way for me on Linux)
The link you posted is a ‘Google sharing’ link and it needs a Google account login to see your files.
You should use a ‘Share with everyone’ (or whatever it’s called) link to allow anyone to download without a requirement to log in.
if you have been telling me to configure the pen
I can say that I’ve already configured it but the problem is that krita “doesn’t recognize”, it always switches to another tool but it doesn’t undo or redo the stroke
You have set the pen buttons to actions. From my experience Krita doesn’t understand actions (must be a limitation of Qt). To make it work you have to map the pen buttons to the actual keystrokes that are configured in Krita which should be ctrl + z for undo and ctrl + shift + z for redo if you didn’t change it. I had a similar issue with my Wacom remote but setting the buttons to the actual key combinations worked for me.
Glad it worked. Advantage of this method is you could map pretty much any function of Krita to the buttons. Of course putting other things on there also prevents you from using the pop-up palette, opening the context menu and zooming/panning with the pen but it all depends on what you use most often.
Hello and welcome to the forum 
