Translation:I have a bad headache now, it must be a cold.
I does not accept it for me, I wrote "I have a headache right now, I must have caught a cold".
According to their own advice 一定是感冒了 can mean "caught a cold". Just hover over the Chinese characters with your mouse and it says that.
This is dumb, it seems we are in the business of learning English here, not Chinese. The only way to get the questions all correct, is to remember the English answers by memory. How that fits with effective learning I have no idea.
And how many years will go past before they do anything about it?
I know it is free but this is always irritating. And once you start the course and want to finish it as I do, it is a continual problem. Getting marked wrong for something you know you have got right.
If I understand correctly, there is a confusing resemblance between this and previous sentences.
We have previously seen sentences where 感冒 means to catch a cold, but that is not what is going on here.
We have previously seen sentences that go Subject Predicate comma Predicate, like 我觉得不好，不要出去, where the second predicate has the same subject as the first. But that's not what's going on here either.
In 一定是感冒了, the predicate is 是感冒. We know that 是 forms predicates out of nouns, so 感冒 has to be the noun "a cold" rather than the verb "catch a cold." And if the predicate is "be a cold," not "catch a cold," then the implied subject is not 我. It's the headache, or rather the disease that the headache is part of (part standing for whole is called a metonym). [Edit: or it's an impersonal clause, where in English we would use the dummy subject "it," as suggested here.]
If 是 has some secret adverbial use that I didn't know about, I could be all wrong about this. But I don't see any use here that matches this sentence.
The hover hints are extra unhelpful in that they make it look like 了 is part of "catch a cold." In 我感冒了 it marks that the change of situation, from health to having a cold, is currently relevant. In 一定是感冒了 it marks a relevant change in understanding about the situation: "Oh! The headache is a cold symptom." Duolingo has some kind of automated system for aligning words and phrases of English and Chinese, which made a mistake because it saw 了 right after 感冒 too many times and assumed they were all one word.
I don't know why there is a 是. I have had many colds this year (感冒, and no one ever uses 是 when they tell me that i have a cold again or am getting a cold or any other thing that people say when you are coughing a lot or don't feel well. btw, 感冒 does not mean the same thing as when we say "I have a cold" in English. You can be 感冒了 without any congestion, whereas when you have a cold, congestion is a required symptom.