I can't activate my iphone

Why can't you activate your phone? Activation Lock or some other reason.

Activation Lock.

You need the Apple ID and password that was used when the device was first set up. If you don’t know that, you can’t remove the Activation Lock.

If this is a used device, the previous owner needs to go to https://www.icloud.com/find/ and erase the device, then remove it from their account. If they won't do that, the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.   *

If you are the original owner, contact Apple. Apple can remove the ID  if provided the required paperwork to include the original purchase receipt with the serial number on it. 

Request additional support for unlocking your device.    -

You used to be able to take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened) and have them remove the ID. This may have changed, so call the store before going to make sure they can help.

Genius Bar Reservation and Apple Support Options - Apple.   -