Mogen vs Gern Haben
There are times when the system will accept "Ich habe es gern" as well as "Ich mag es," for example. Then there are countless times when it will not. These terms are synonymous and interchangeable. The system should accept both.
As I understand it, alternate sentences are provided first by course creators, and then later via suggestions from course students. I don't know any German, but I surmise your suggestions are phrases, right? Not whole sentences? Because context can change the meaning of a word or phrase, the system can't just globally add the synonyms that worked in sentence A to also work in B, C, and D. You have to add those suggestions each time.
When you complete a question, read the discussion. A commenter might explain why this time, the phrase isn't actually appropriate. I see this a lot in the Spanish forums: there are so many situations where it seems that a rule applies...and it doesn't. I actually suggest you read the sentence discussion often anyway, because it can be so informative.
Then, if you still think that your sentence is an appropriate translation, click "report an error" and say that your translation should be accepted.
These reports are handled manually as far as I can tell. I suspect what's going on here is that the phrase has been added to sentences A and B, but not C and D because the moderators have not sifted through those reports yet.