Translation:Today's dinner is not delicious at all.
I'm not quite sure I understand what you're asking. Are you saying that instead of the word ご飯 (gohan), if you drop the ご and just write 飯 it will be pronounced as めし (meshi)? Yes, that's right. Usually a kanji standing on its own takes the kun (Japanese) reading.
Or are you saying that 夕飯 should be pronounced ゆうめし (yuumeshi) because there's no hiragana? In that case, ゆうめし is a potential alternative reading, but ゆうはん is more common in my experience. Two or more kanji combined can take either the kun (Japanese) reading or the on (Chinese) reading, regardless of whether or not there is hiragana in the sentence.
Getting tired of the fact that "today's dinner is not tasty at all" is marked as correct but "today's dinner is not at all tasty" is marked as wrong. Have reported it multiple times but no joy. It's hard enough to remember the meaning without having to remember random choices of word order. :-(
の is helping 今日 modify/describe ゆうはん. It helps nouns modify/describe other nouns and it also shows possession - either of which could work here today's dinner - the dinner "belonging" to today or today as a descriptor describing which dinner we're talking about. I get the feeling that you are maybe under the impression that の and は are interchangeable - they are not.
1) Was it a normal "type this in Japanese" question? Sometimes unusual kanji/kana combinations haven't been added to the database, so all you need to do is submit an error report and it should be added eventually.
2) Was it a "type what you hear" question? There's an issue with that type of question where only one specific combination of kanji and kana is accepted.
I always answer "oishii" questions using "good" and have never had a problem. Always make sure to check if you had any typos or accidentally put the verb in the wrong tense, etc. If your answer was word-for-word exactly the same as "Today's dinner is not delicious at all", just replacing delicious with "good", then submit an error report and I'm sure the answer will be added.