It's wrong as in it's unnatural to phrase it like that:
"Den er en meny" is equivalent to "Den menyen er en meny". If you already know what it is, explaining it is redundant. 'det' is preferred as it doesn't refer to anything.
'den' is only used if you're referring to something you've already talked about.
What you're likely hearing is the "r" in "er". The "t" in "det" is silent, so there should be no consonant sound at the end of it at all, making the pronunciation sound more like "deh".
"Der" is pronounced with the "r", but also has more of an "æ" than an "e" sound for the vowel.
So, we need to put you on the intensive Norwegian ear training. First, you need to get some cold fjord water and use it to clear your ear canals. Then, you ought to hike north and at the first sight of northern lights, you need to start uttering the following words very rapidly. Det, de, det, de, det, de, det, de like some kind of a mantra. After that, an old woman representing the Norwegian language will appear behind and give you her MP3 player with the same song. After it finishes playing, you'll be cured and you'll never ever confuse these two words again.
Of course, this is me joking. Please don't use salt water to wash your ear canals!
Don't worry, it's normal to have some difficulties in differentiating these two when you haven't been exposed to Norwegian prior to choosing it in Duolingo. Click the Forvo links for det and de above. Create an account there (you can use Google or Facebook) and download the sound files. Then, ask someone to play them for you without telling you which one is it. Do this several times a day for one week and then tell us if it helped.
The two vowels that are causing you linguistic pain are the mid central vowel and the close front unrounded vowel. Follow this two Wikipedia links and click the audio samples to hear what they should sound like.