why on this app can I not attack with the king???

The king can never step into a square that is attacked by an opponent's piece or take a pice that is protected.

You're putting yourself in check. Mr. Stahl is a moderator here and a great guy. He's pretty busy so you can be happy he answered you. It's people like him that make this a great site. The King, in general, shouldn't be having to attack anything until the end game.

Don't give up hope. Run through some of the lessons, they are excellent and will give you a feel for the game. It's tough learning this game. It is the most infuriatingly complicated game ever devised. You absolutely have to study in order to play well. You also have a huge community here that want to see you succeed and will give you no end of advice and help. They can also be a little harsh, but, hopefully, it's all in fun. Reading through the forums and old posts can teach you quite a lot. Check out the Beginners forum. Welcome to the site. We're glad you're here. Good luck to you.

It might be beneficial for readers if you post the relevant position.

In any case, Martin and pcwildman are correct.