Does a spell that says 'counter target spell' work on a mana ability?

A mana ability is defined by three criteria (CR 605.1): it's an activated or triggered ability, it could produce mana as part of its effect, and it's not a loyalty ability. Crucially, mana abilities do not go on the stack (CR 605.3a) and resolve immediately when activated.

Because mana abilities never exist on the stack, they are never 'spells' or even 'abilities on the stack' that can be targeted. A spell that says 'counter target spell' can only target objects that are spells on the stack (CR 114.1a). A mana ability is neither, so it is an illegal target entirely.

Similarly, even effects that say 'counter target activated ability' or 'counter target triggered ability' cannot counter a mana ability — CR 605.3b explicitly states that mana abilities can't be countered.

Concrete example: Your opponent taps a Forest to activate its mana ability, producing one green mana. You cannot cast Counterspell (which counters 'target spell') in response, because the Forest's mana ability never went on the stack. The mana is produced immediately and your Counterspell has no legal target.

If an ability is not a mana ability — for example, it produces mana but also does something else, like Selvala, Heart of the Wilds — it does use the stack and can potentially be countered by an appropriate 'counter target activated ability' effect, but still not by 'counter target spell.'