Chosen Solution
Recently replaced a screen on an iPhone X for a customer. Everything is fine except Face ID, phone saying Face ID not available. I know the ribbons are very delicate so was v careful (have done this repair before) and there are no signs of damage to the ribbon. Any ideas/suggestions?
Perhaps there has been a (blue?) little scratch-protection-sticker in the spare display which you forgot to remove?
Face ID is much more complicated than you think. Why Face ID Not Working After Repair
