If a land enters the battlefield tapped, can you still tap it for mana the same turn using an untap ability?

When a land enters the battlefield tapped, it simply has the tapped status and cannot be tapped again until it is untapped. However, nothing prevents you from using an activated or triggered untap ability to untap it during the same turn it entered — and once untapped, it is free to be tapped for mana like any other land.

The rule that prevents you from tapping a land for mana the same turn it enters (the 'summoning sickness' rule, CR 302.6) applies only to creatures. Lands have no such restriction. The only barrier here is the land's tapped status, and that barrier is removed the moment an untap effect resolves (CR 701.18).

Activated untap abilities (written as '{Q}: …' using the untap symbol, CR 702.2c) are a common way to accomplish this. Similarly, a spell or ability that says 'untap target land' works just as well.

Example: You play Aether Hub, which enters tapped. Your opponent controls a Kiora's Follower ('{T}: Untap target permanent'). If you control Kiora's Follower yourself, you can tap it to untap Aether Hub, then tap Aether Hub for mana — all in the same turn it entered.

So the condition is simply: you need a source that can untap the land. Without such an ability, you must wait for your next untap step (CR 502.1). With one, you can generate mana from it right away.