You need the present tense marker in this because it is not negative.
U- = You -na- = Present tense -andika = Write
That would be a different (rarely used - only in newspaper article headings; nearly.) tense.
That tense still adds an extra a though, so
waandika = you/they write
mwaandika = you (pl) write
twaandika = we write
I think there's a bug in the system because when i pick the right answer, it refuses to accept. hope they fix it.