Can't grant Input Monitoring permission to Wacom drivers on macOS Catalina
Unfortunately this problem affects everyone installing Wacom drivers on macOS Catalina, but at least there is a workaround. (Skip to Workaround at the end of this answer if you don't care about the explanation.)



How it's supposed to work

The way it's supposed to work on macOS Catalina is that you receive a permission prompt (like this one shown below) when you first use an application that needs to use Input Monitoring. But for some reason, the Wacom drivers never cause this dialog to appear.



Why Wacom's instructions don't work

Wacom has at least acknowledged the issue, but as you have discovered their instructions are incomplete! They say to drag WacomTabletDriver and WacomTouchDriver into the Input Monitoring list, but that's impossible on a stock installation of macOS Catalina because the list isn't even shown.

The list will never be shown, and the + button will be missing, until some other application requests Input Monitoring permissions.

Workaround

You'll need to install and activate at least one other application that can use Input Monitoring before you can finish setting up the Wacom drivers. Once you have the Wacom drivers working, you can remove any other applications you don't want to keep using. Here are some options:


CheatSheet is a small free utility program that's easy to install, and it's easy to quit/uninstall afterward.
ColorSnapper2 is another option. It's not free, but the trial version is enough to get the Input Monitoring prompt.
Discord - A popular chat platform.
Klokki.app - I've seen others use this.
BetterTouchTool can create the AEServer entry that is shown in the Wacom instructions.
Karabiner-Elements helps to configure custom key bindings.


Other cases

You're not the only person to be affected. I found a couple other cases here on Ask Different: 1 2 3 4. And on some other sites like Reddit and Apple Communities.