Translation:The curtain of your room is blue.
Since so many sentences in duolingo require a more direct translation even if it sounds weird in English (eg, 'the salad has salt'), I wrote 'the curtain of your room'. Turns out in this one the rule is different. For consistency, both direct and 'improved' translations should be accepted.
Well, normally, this sentence would directly translate to "The curtain of your room is blue." This is what του stands for. I guess it would be a bit of an awkward sentence in English, so the translation was not word-to-word.
Του δωματίου is genitive, and it expresses possession.
In that case you need to submit a bug report here https://support.duolingo.com/hc/en-us/requests/new In the future, please remember to take a screenshot to attach to your bug report.
The course contributors cannot do anything when the system does not accept answers that are already in the database; it's down to Duolingo staff to figure out why the system is misbehaving.