Can you activate mana abilities during the untap step?

The untap step (the very first part of each turn) is a special phase where no player receives priority. Because players never receive priority during the untap step, they cannot cast spells, activate abilities, or activate mana abilities at that time. This is stated in CR 502.3: 'No player receives priority during the untap step, so no spells can be cast or other nonmana abilities can be activated.'

Mana abilities are a special category of activated abilities that normally can be activated any time you could activate an activated ability (CR 605.3b). However, since no priority is ever granted during the untap step, even this special permission for mana abilities does not apply. You simply cannot act at all during the untap step.

It is worth noting that some effects that trigger or apply during the untap step (such as phasing or untapping permanents with the 'untap' symbol) happen automatically as part of the step itself — but these are not player-activated actions.

Concrete example: You control a Llanowar Elves (which has a tap ability to produce green mana). At the very start of your turn during the untap step, you cannot tap Llanowar Elves for mana — you must wait until at least the upkeep step, when you first receive priority, to activate any mana ability.