Why can’t i put face ID

The “best answer” was on the page you left to come here. The link is at the top of the page. I’ve copied the pertinent portion here:

1. Update your iPhone or iPad to the latest version of iOS or iPadOS.
2. Check if there is an issue with Face ID in your iPhone's parts and service history. To do this, open the Settings app, tap General, then tap About.
3. If you still 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.

Only a trained technician should service your device. Improper repair, modification, or use of nongenuine Apple components in the laser systems might prevent the safety mechanisms from functioning properly and could cause hazardous exposure and injury to eyes or skin.