Calm down. No need for multiple question marks.
No. If es acts as a demonstrative pronoun (aka meaning this/that) then it can be used for objects of any gender. If you said "Er ist ein Schlüssel", then you'd be referring to a human male.
Also: Never leave the umlaute out. If you don't have them on your keyboard, put an "e" after the vowel. This is the official rule on how to write äöü if it's not possible. So "Schluessel", not "Schlussel". It is quite important to include them as they can sometimes drastically change the meaning of a sentence.
Getting a German keyboard on a Mac (and learning how to switch between keyboards) can be done in ten or fifteen minutes. Probably the same for PCs. Unfortunately Duo is very lenient about all punctuation. Since adding an e after the vowel is a legit way to create an umlaut, I suggest that contributors insist on it. In other words, ein Schluessel should be accepted, but ein Schlussel should not. That way we would develop correct habits, not to mention pronunciation.
Unless you're having a conversation about a key and you used "Es ist ein Schlüssel." as a stand alone sentence/thought, it's fine, "es" can be used.
If you are saying multiple thoughts/sentences about the same key to describe a certain key to someone, the demonstrative pronoun would change to "er".
Zum Beispiel (an example): Das ist ein Schlüssel. Er ist klein und gelb. Er is nützlich.
Or you can go to settings and have a German keyboard available. A keyboard icon will appear in the main menu, next to the time and day. Then you can toggle between keyboards. Google how to do it, it is easy. I had to install one for Russian, but now I use my Spanish keyboard all the time.