Here are the completed sentences with suitable conjunctive adverbs:

1. I want to buy this fashionable dress; **however**, it is too expensive.
2. This restaurant is cheap and famous; **therefore**, we need to reserve a table as soon as possible.
3. You should write the essay more quickly; **otherwise**, you won't submit it on time.
4. My son is addicted to computer games; **however**, I only allow him to use his computer or mobile phones only two hours a day.
5. Remember to put the milk in the fridge; **otherwise**, it will be sour and we can’t drink it.
6. Barbara is not hard-working at school; **therefore**, she always gets good marks in the exams.

(Note: For sentence 6, you may alternatively fill it with **however** or keep it as is, but that would slightly change the meaning.)