Can you tap a land for mana during the untap step?

The untap step is unique in that no player ever receives priority during it (CR 502.3). Because you can only take actions — including activating mana abilities like tapping a land — when you have priority, tapping a land for mana during the untap step is simply not possible.

Mana abilities can generally be activated any time you could take a special action or whenever you have priority (CR 605.3b), but that flexibility still requires you to actually have priority first. The untap step never grants priority to any player, so that window never opens.

After the untap step ends, the game moves to the upkeep step, at which point the active player does receive priority (CR 503.1). That is the earliest point in a turn where you could legally tap a land for mana.

Concrete example: It is your turn and your Breeding Pool enters your untap step tapped. Even though it is about to untap, you cannot tap it for {G} or {U} before it untaps — you must wait until at least the upkeep step when you first receive priority.

Relevant rules: CR 502.3 (no priority in untap step), CR 503.1 (priority in upkeep), CR 605.3b (when mana abilities can be activated).