Activation Required come up on my phone

Is this a new problem or did you recently purchase this from someone other than Apple? If you recently purchased this iPhone and see this message, then contact the seller. They will have to go thru the steps below to remove the code:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

This link may help:

If you can't activate your iPhone or iPad (Wi-Fi + Cellular) - Apple Support