Ok, I'm going to argue that this is one of the cases where it's more natural to change the verb tense in translation.
So I would suggest the more natural translation is: "At first, I wasn't sure". The given translation - "In the beginning, I am not sure" - sounds quite odd as an English sentence.
What do others think?
Yes, they should accept the far more natural "I was not/wasn't sure" since it is clearly referencing a past action.
Newspaper headlines oftentimes reference past actions, using present tense. I hear there even exists something called Historic Present, so je ne suis pas sûr
Correct, but if given "Au début, je ne suis pas sûre.", it should be valid to translate it as both present or past. Duo does not give context and without it both are valid.
What about present perfect - is it OK in English (I know it is not considered OK by Duo, but I ask whether I should report it or not)?
One of the drop down options was for "sûr"...which is what I chose....and it was marked wrong...?