solution #1
make sure you’re connected to a functioning wi-fi network that is able to connect to the broader internet. Sometimes a very restrictive router can be the culprit and may block the iPhone from attempting to reach Apple’s activation servers, so simply joining a functioning network does the trick, allowing the iPhone to communicate with the outside world and skipping the screen.
Solution #2
You can bypass Activation errors by using any original carrier SIM card, it doesn’t need to be for an active account or a valid phone number – so even if you or a coworker has an old one sitting around in an office desk somewhere, it will work to pass activation. It just needs to contact the activation server, verify that it’s good (ie: not stolen) and then you’re good to go.