What version of the software are you on? Apple disabled iPhone 8 with DIY screen replacements in IOS 11.3. After a public outcry they then relased a further update to enable the phones again. Use iTunes to update to the latest version and all should be well.
( https://www.express.co.uk/life-style/science-technology/944736/Apple-iPhone-screen-repair-iOS-11-3-update-iPhone-8 )