The front facing camera of the Galaxy S21 Plus is secured in place with an adhesive seal. It is not mandatory to replace this seal however the front facing camera will have a little amount of clearance if you don't.

Prepare an iOpener and apply it to the screen where the front facing camera is located for at least two minutes to loosen the adhesive underneath.

A hair dryer, heat gun, or hot plate may also be used, but be careful not to overheat the phone—the display and internal battery are both susceptible to heat damage.