How to enable Face ID on iPhone XS?

Follow these steps, testing after each step:

• Face ID not working.
• Software Update.
• Turn iPhone on or off.
• Force Restart iPhone.

If that didn’t work, then it could be a hardware issue.

According to Apple:

Using diagnostics available in iOS 14.6 or later, Apple has detected an issue with your TrueDepth camera that might cause it to not function as expected. The TrueDepth 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.

As a result, I recommend service.

Service Checklist:

• iPhone service pricing.
• Check AppleCare Coverage.
• Get your iPhone ready for service.

Click ▶︎ Start a Service Request. ⚛️

Need more help?

• Chat now or call 1-800-APLCARE.
• International Support.

Click ▶︎ Check Repair Status. 🔧

You didn't provide much information, but I assume you can't activate Face ID.

Here's how to Set up Face ID on iPhone - Apple Support

Please check this article If Face ID isn't working on your iPhone or iPad Pro - Apple Support