It's OK, I meant that ''Bạn sai rồi'' is better'. I think it's a bit rude to say ''Bạn sai''.
The problem is that rồi will be somewhere later in the lessons. They couldn't introduce it here.
Amusingly, sai is Malaysian for ‘❤❤❤❤’, hence the Singaporean phrase ‘kena sai’ (lit ‘encounter ❤❤❤❤’, i.e. get screwed). So you’re either dung or sai.