"František is not at home today."
Translation:František dnes není doma.
Because it sounds strange. Even when strongly stressing "dnes" it is still strange. But I don't which specific rule would be against it. The word order and especially the position of adverbs is different in Czech and in English.
Seeing the number of reports demanding it - it is a GOOD thing we do not accept it. Czech is different. Do not force the English word order here. It just isn't natural. This position would only work for a very strange corner case with a very special meaning.