aren't we supposed to put den instead of det because nyckel is an "en" word
Nope, we always use det at the start of a sentence when we're introducing something that hasn't been mentioned before.
Nyckel is just the door key or may be any other key, like the key to her heart or the key word
It can also be used for key word and similar analogies to a door key. But it cannot be used for e.g. the key of a song or a key on a keyboard. That would be tonart and tangent [tanjent] respectively.
The short Y and the short I are not pronounced the same way, so there is no confusion.
There is a bug which causes correct translations not to be accepted, even when copy-pasted. Unfortunately, the only fix I know of is to restart the lesson.
So how about. "Det här är min nyckel." Or "Det där är min nyckel" or is it okey to leave out where the key is?