"I will quit the company tomorrow."
In an informal setting yes 'yameru' would suffice.
Duolingo so far in my experience has not been using informals like that.
Plain form works on most sentences where you can form ru-verbs
They should allow both informal and formal already. Informal is already allowed for some translations, plus it's good practice to know both! And I hate it when the red shows up because it makes me doubt the Japanese I already know! /rant over/
IMO, やめる should work, but give you a little comment like "informal language" or something.
Exactly. Given that やめる is among the tiles being offered, this way of doing it punishes recognition. I say, "Oh there it is" but instead of fluently selecting it, I'm supposed to think, Oh, maybe they want me to look for something different. Oy. Besides, who FORMALLY discusses their job-quitting plans?? This is the kind of sentence that is only uttered among trusted friends.