The correct answer is: B. didn’t/could have had
The sentence is in the past tense, so we need to use past tense verbs. The first blank is asking for a reason why the person didn’t come, so we need a verb that expresses regret or disappointment. The second blank is asking what would have happened if the person had come, so we need a verb that expresses possibility or potential.
Option A is incorrect because “don’t” is in the present tense. Option C is incorrect because “will” is in the future tense. Option D is incorrect because “doesn’t” is in the present tense and “will have” is in the future tense.
The correct answer is B. didn’t/could have had. This sentence expresses regret or disappointment that the person didn’t come, and it also expresses the possibility or potential that they could have had a good time if they had come.