Hey, I think you may be having the same issue as I was just having.
If you have a new outlook.com account or have recently changed your password on your outlook.com account you probably have Two-Step verification turned on and your iOS device needs the unique password that Outlook.com generates so it knows the device is approved by you to use with your account.
Login to Outlook.com on a computer and look under Account Settings>Security Info, from here you have two alternatives.
1) Less Secure Option --> Turn Off two-step verfication and use your regular password on your iOS device.
2) More Secure Option --> Turn Off two-step verification and then turn it back on again. you will see the link "I sync my Outlook.com email on an iPhone" click this and follow the instructions. You will be given a unique password to enter on your iOS device. Enter it and try logging in again.
I hope this works for you, it fixed my issue.