I know right?! What the heck?

I was about to say the same thing !

The suggested answer (the one you see on top of this page) should of course always be accepted. If it isn't, there's a bug somehow. If someone can share a screenshot of this with us, we can report it to Duo staff.

There's a known problem with spelling variation for i morgon and imorgon where it says Imorgon is a typo (it isn't, that's also correct) that we have already reported, so we don't want screenshots of that. But if you don't get the literal suggested answer accepted, please share a screenshot.

Forum post about i dag/idag bug: https://www.duolingo.com/comment/12941839

The future tense kommer att can be used, especially in the spoken language, without att. This is getting more and more common.

Definitely. Both spellings are just as fine. Traditionally, "i morgon" would be considered a bit more correct, but unless you're publishing a very serious book or writing for an esteemed newspaper, nobody is going to bother - and probably not even then.

It's accepted and common, but to spell it with the space is recommended.