"She walks around."
Translation:Ella camina alrededor.
Indeed, this is wrong on Duolingo's part. "Ella camina alrededor" means "She walks around __"; without filling in that blank the thought is incomplete. To get an accurate translation you'd have to change the sentence structure quite a bit with something like "Ella va caminando por ahí". The issue with "andar" is that it doesn't necessarily imply "walking" as much as general moving or going. Also, "Ella anda" is likewise an incomplete thought as it doesn't give enough information about what's going on.
Were you thinking of "cerca" with an "e" not an "i"?
I think cerca is more for walking around something, meaning walking near to it, which is a little different from just walking around. And in this context I think it's actually "cerca de". Eg: ella camina cerca de la casa.
Circa means around or near. It is used in the scientific literature to express that, although the certainty necessary to provide a unique value is lacking, other reliable data exist that allow to assure that the fact or data that is communicated in the text occurred at a moment -or had a Value-anterior or posterior to that cited in the reference scale used, but always relatively close to the one that is transcribed.