Where the heck is the eraser tool?

Krita has eraser brush presets, not a separate eraser. See https://docs.krita.org/Introduction_to_ ... aint_tools

Krita also has an eraser button on the toolbar. It is a toggle, so you can switch between drawing and erasing. Erasing is pretty common for a lot of people, so the toggle button is assigned to the "E" key.

Many thanks, guys. As you can tell it's day one.
Just one thing, I'm erasing but it only removes, say, half of the mark. As if it's leaving a stain behind. Am I doing something wrong?

Well, since it's a normal brush, normal pressure curves apply. Search for the "hard eraser" to get an eraser that erases without mercy.