Activation problems are almost always caused because the phone was hacked to unlock it. Contact the carrier it is locked to and inquire as to a legitimate unlock. If they will not do so, you will not be able to activate that phone at all. It's a brick.
The other likely possibility is that you do not have a SIM card installed in the phone at all. If that's the case, insert a SIM. You can not activate without one.
Less likely is the possibility that the activation servers are temporarily down. It's rare, but it does happen. This is usually resolved within a few minutes.