Translation:If you cannot come, then you definitely have to tell us.
"If you can't come, then you must tell us" should be the correct answer. 如果...就... is an if...then... structure.
That is too literal a translation (although not wrong either). In English, it is perfectly fine to use the conditional without a 'then' to introduce the resulting clause.
It is still correct to have it in, though. Neither one is wrong. That's the point. That and the fact that, at times this site's Chinese is hyper-literal, and at times it is more free.
Got penalised for putting "Then" in an "If...Then" sentence structure. Answer needs to be fixed to permit it.
If you (cannot/can't) (come/make it), (-/then) you (definitely have to/must) (let us know/tell us). These are all acceptable translations as in English, generally the "then" is implied and it is not colloquial to have it here.
If you cannot come, then you have to definitely tell us. Reported 3.23.19
You should tell us if you don't come.. Should be a correct answer.. Duo is really stupid... Can you fix this ASAP. Don't know how many people are there working on it....