Yes, it is frustrating, and it is hard to know what search terms to use to find the problem.

There is a known issue when you combine Wacom devices with "Windows ink" functionality. It affects many programs, not just Unity. It even affects quite a few of Microsoft's own tools, such as the popup keyboard making your tablet's cursor vanish since it stupidly assumes finger-based touchscreen rather than digitizer with stylus.

Wacom says it is Microsoft's fault, here is their workaround.

Alternatively, open your tablet configuration, and make a custom application setting for Unity to force pen input into mouse mode.

Are you sure the tablet takes the mouses place if you use it? If not, the question is how Unity would recognize it. IDK if you can have multiple mice connected at once and use all of them at the same time with Unity.

I know its not a problem in the Editor, but for the playtest the controller settings are different, your ingame code now needs to take care of handling controllers.

You might need to check

a) does your tablet appear in the list of joysticks? (haven't tried that, my wacom tablet connected to the PC where I do playtests is usually off, but I have all kind of weird and wonderful controller devices connected, and they all appear as joysticks)

b) what happens when disconnect the mouse and make sure the tablet is recognized as a mouse? (might need some driver tweaking, IDK)

Yup like, while on Playtest I can't move the camera around with my Tablet's pen, so without disconecting my tablet (I just put the pen down) I turn on the wireless mouse and I'm able to move the camera, and then without turning off the wireless mouse I try to check if now the pen works, but it still doesn't, Gian-Reto can you move the Playtest camera using your tablet?

a) I don't know what you mean by this, since my PC its in spanish and Joystick its translated literally to "Palanca de Mando" and there's no such thing named like that I have no idea, if that's like the drivers both my Tablet's Pen and Wireless Mouse are listed as "Mouse" in properties.

b) doesn't works, still can't move the camera during a Playtest

Ok, first things first:

The "playtest tab" basically lets you play a playtest build of your game. That means Input is handled by Unitys ingame Input controller classes.

Which is fundamentally different to what you have in the normal editor. There, the mouse is "hardwired" to do certain things, as is the keyboard, as is the tablet, and so on. Joysticks / Joypads / Flightwheels /all other analog input devices most probably don't work at all in the editor as long as you don't get a special plugin or write your own editor script that makes the input device work in the editor (I got a special free plugin to make my Space NAvigator work in the Unity editor, and its brilliant...)

All this needs to be done by your own scripts in your game builds, including the playtest build that gets automatically generated for you for use in the "playtest tab" of the editor. I guess you attached a mouse controller script to your camera. This controller will listen specifically for mouse input.

Your tablet however, as long as you don't have some special driver settings running, will not be registered as a mouse... as it is, really, a completly different input device. That is the reason why a mouse controller script that is just listening for mouse events will never pick up your tablet events, as long as you do not go in and edit the script to listen for the tablets Input events.

IF the tablet can be used for ingame input, IDK. That was why I suggested to check what "Joystick devices" are visible to Unity (just look it up on the english part of the unity API, Unity will provide you a list of all the Joystick devices it found (Input.GetJoystickNames)... your tablet MIGHT be there (altough IDK really)...

Another thing you could check would be for special drivers to include in your game builds.... these are for example needed to get the XBOX gamepads running with DirectInput, which is the only way to get rumble output working (the GamePads work fine with Unitys built in Input manager, but without the possibility of rumble output).