tournament blinds 6k/12k

utg all in for 20k

btn call

sb fold

can bb raise? he hasn't acted yet but what is the actual rule?

preflop btw if that wasnt apparent

Yes, BB can raise.

Of course
Why is this even a question?

because if it was on the flop the big blind wouldn't be able to raise

Why do you think that?

The important thing for you to remember is that the issue is not does an all in bet close the action the issue is does it reopen action. Players who have not acted on the last legal bet or raise will still be able to act regardless of the all in the only question is whether an all in reopens the action for the players whose action had already been closed.

So in your first post the BB had not yet acted his action was still open so it remains open. In the second case you present (after the flop) BB had checked so his action was closed. But the all in was big enough to be a legal bet so it reopens the action.

(Caveat: in limit where there is a betting cap on the number of raises it is possible that an all in bet caps the betting and therefore closes the action for everyone)

I think OP is saying post flop the BB leads out for 12k followed by an all-in for 20k and a call.

While that may be what he is thinking ... It certainly wasn't what he was saying.

As indicated, when a Player 'has acted' on any given street then they are restricted by the 100% raise rule, which prevents them from reopening action they already created (or agreed to by calling). But if any Player has yet to act on an active street, then they get their 'full' opportunity to do so.

This would also be true in a cash game if a Player had posted a missed Blind behind the all-in. GL

Question-does a bet less than 1 bb open action post flop? For example, first to act checks, second to act checks, third player goes all in for .2 big blinds. Can first to act raise at this point?

No, any 'checking' Player is only allowed to call unless someone behind the all-in makes a full raise ... which is usually the question in that spot. A Player must 'raise' (open) to 1.2 BB, NOT 1.0 BB.

There may be a quirky rule in Limit Poker that requires 'calling' Players to put in a full bet ... not sure.

If PF, then all 'calling' Players must put in a 'complete' BB if the BB is all-in for less than a full BB. GL

No but in limit play an all in that is at least 50% of the bet is considered a legal bet or raise. So if playing 50 100 limit on the flop a player checks and the next player goes all in for 25 that is a raise and reopens the action. If that happened on the turn it would not be a raise and it would not reopen action .