Should be correct. Maybe the exclamation mark is Duo's way of saying that it wants us to use the special response to 'Shikamoo', the respectful way of greeting someone older. I'm just guessing. But anyway, asante does mean thank you, so should be correct too
Shikamoo is a greeting for elders, marahaba is the response. It is not used as thanks in any other context. Asante is the correct answer.
Agreeing with ElDoctr above, I've only ever encountered Marahaba as a specific response to Shikamoo. Thank you as a stand alone phrase should be translated as Asante, so duolingo is misleading in this instance.
Why isn't this translation 2 way? I translated Marahaba (the word before this!) as thank you and was marked incorrect. Please be consistent!!!
Marahaba is only used in reply to shikamoo. As such for an English speaker to translate it as thanks and try to use it as such would be a problem.