Can Janna, Savior be played from hand to battlefield where there is an ally unit and an enemy unit? The opponent moved their unit to my battlefield where I have a unit.

Yes — you can play Janna, Savior from hand straight into that battlefield. Reaction gives you the timing (any time, including a Closed State), and the location is legal because it's a battlefield you control: the opponent moving in only makes it Contested, and control can't change while the Combat/Showdown is ongoing (Rule 190.4.b). The "can't play to a contested battlefield" limit only bites the attacker, who never controls the field they're attacking.

So her enter trigger resolves in your favour: heal your units there, then move up to one enemy unit from there to its base (the "up to" is optional, so you can decline). That strips the combat of attackers, so it resolves without a Combat Damage step and you take nothing.

One critical timing note: the trigger goes onto the Chain after she finalizes, so the opponent gets priority to respond before the attacker is moved (they could, for example, Ambush another unit in). And this only works while that battlefield is still yours — if they've already conquered it, she'd have to be played to your Base instead.