When I got it wrong, it was because I didn't include "at all" in the translation. "Yet" is definitely accepted, so perhaps there was something else in your sentence that Duo didn't like. For troubleshooting, it's helpful if you use the Report button, because then we can see your complete answer.
Why is this 'has been' and not 'was'? It is (default) imperfective, right? Also, 'been' suggests a verb of motion in this context. (Where did he go today? He has not been anywhere at all.) I am a bit slow picking this up. (I think of the past imperfect in french and italian. Equals present perfect in english??)