It’s correct. This course is still in beta, so it sometimes doesn’t accept correct variants because the authors didn’t think about them. Next time you get this question, please report this error to the course creators by clicking on 'My answer should be accepted'.
"I am going there sometimes" doesn't really work because of the "sometimes", it changes the sentence. In this case, you wouldn't use the present tense "going", you'd just say "I go there sometimes" (or "I sometimes go there" or "sometimes I go there"). Does that make sense?
- туда = there, to that place,
- куда? = where?, to what place?
Aside from being an interrogative pronoun, куда can also be used to connect clauses in a complex sentence:
- Она знает, куда я хожу. She knows where I go.
Is it "walking there" or "walking to there" because if it is not "walking to there" this changes everything. "Walking there" and " Going there" have very different meanings.