What version of iOS are you using?
There was an update that fixed that issue 11.1.2. The current version is 11.2.1.
Go to Settings > General > Software Update to check if there is an update available.
Make sure you're connected to wifi unless you have a generous mobile data allowance and have sufficient battery left >50%. Better to plug-in to the supplied charger when doing a software update.
Question: i have an 8+, and since it came out of the box i have had issues with the speakers cracking at all volume levels. is there anything that can be done to repair or replace the hardware associated with this issue?