Technically, the tense should match, so since we're using the perfect tense (conditional perfect in fact), you should use was.
But both could have ... is and could have ... was are acceptable in this case.
Why? Well, it's because your clause about the paprika being similar to the bell pepper might be true for a long time, and might continue to be true even in the present, so it might be okay to use is.
Maybe not. Maybe the paprika is rotten by now, or already eaten. Then is would make no sense. But a situation continuing to the present would justify use of the present tense is.
Consider:
"I could have gone shopping, because the supermarket was near."
"I could have gone shopping, because the supermarket is near."
If the supermarket has not suddenly moved, it probably still is near – it is now, and it was then – so either one is acceptable.
But some things do not last so long:
Correct: "I could have stayed longer, because it was early in the morning."
Incorrect: "I could have stayed longer, because it is early in the morning."
Unless you are describing something in the very recent past (minutes or hours ago), the fact that it is now early probably has nothing to do with the situation in the first half of that sentence, so mixing the past and present tense in this last example doesn't really work.
So to distill the relevant portion of the canonical post (which I still recommend you read):
The use of the past or present perfect implies the action happens relative to something else. The past perfect suggests the action occurs before some other action (which you will subsequently mention) while the present perfect implies the action is still going on.
The phrase "It has just finished" adds a little more emphasis to the just than "it just finished", since it implies the action was going on right up to the present moment. Otherwise the meaning is much the same. Feel free to use either structure as feels appropriate.
That being said, "it has just finished a few days ago" is an odd construction. It should be "it had just finished a few days ago (before something else happened)". The action happened in the past, so (if you are going to use the perfect at all) you should use the past perfect.
I don't think Google is a good resource to judge the popularity of an English phrase since it searches for results with parts of the phrase and not necessarily the entire phrase. Try Googling "it has just finished a few days ago" (with the quotes) and you'll only see three results, two of which are this question. :)
Best Answer
Both sentences are correct to my (native Canadian English) eyes. I do perceive a subtle difference in meaning between the two, however: using the perfective "have" strengthens the sense in which the author was not able to finish his novel. That is, I see these two meanings:
The author died before he could finish his novel. He was working on his novel, and before he got around to finishing it, he died. Maybe if he worked harder he would have finished it, or maybe it was just impossible.
The author died before he could have finished his novel. He was working hard on his novel, but he died so soon that it was impossible for him to finish the novel. Even if he was putting his full effort in, he died before the possibility itself existed.
That is, adding the perfective puts an emphasis on the impossibility of finishing the novel, where as without it one leaves unspecified whether, given different circumstances, the author might have been able to finish the novel despite dying so soon.