How do i fix it to let me use face id

Your Face ID True Depth camera may have failed, especially if you have the warning message. 

Using diagnostics available in iOS 14.6 or later, Apple has detected an issue with your True Depth camera that might cause it to not function as expected. The True Depth camera contains lasers, which might be disabled for safety reasons if the device malfunctions or is damaged. This won't prevent you from using your iPhone or iPad, but until your iPhone or iPad is updated or serviced, you might not be able to use Face ID, Animoji, Memoji, and some other features that use the front camera. 

If you get that message and can't enroll your face or unlock your device with Face ID, you can take your device to an Apple Store or Apple Authorized Service Provider, or contact Apple Support about your iPhone or your iPad.  

Click here >  https://getsupport.apple.com/products/choose 

If you had the screen replaced other than by Apple it will never work again 

iPhone parts and service history – Apple Support (UK) 

About genuine iPhone displays – Apple Support (UK) 

See also Knowledge Base article If Face ID isn't working on your iPhone or iPad Pro - Apple Support