１) wrong , I can't keep up with you. 2) right, but it is up not un, Keep up literally means move upward constantly, it can also means you follow somebody tightly, the usage of the first sentence you wrote. The explain just upon my understanding, I can't confirm it is one hundred percent correct.

1) I can't keep up with you! 2) Come on! Keep up (with me)!

I can't keep up with you. Come on, keep up with me!