Why is membuka correct, but buka not?
I typed kita... but it says that is invalid, is this true?
It should have been valid. Kita is synonymous with kami, but less formal.
I used "Kita membuka tas kami" and it's wrong.
"Kita" and "Kami" are both translated as "we" and only differ in formality
Maybe it marked you wrong for using both in the same sentence. I don't know the rules about mixing formal and informal language in the same context, though.