I think/believe she couldn't do that.

Uly, would it be correct to use mustn't? If so, why didn't you?

Because here ”must” expresses an opinion, not a prohibition.
In other words, ”not” here doesn't go with must, it goes with ”not be able to”.

+++Alex

Correct, Olga! It’s like the difference between “She mustn’t be Russian” (if you’re looking for an actress to play a role, but not a Russian one); and “She must not be Russian” (if you tried to speak to her in Russian and she didn’t understand)

so, do we use mustn't only when we express that something is prohibited?

For the most part - yes

Also, MUSTN’T in general isn’t very American - it sounds a little Victorian to us.

good to know, thanks.

👍🏼😉