Hi,

This can be caused by several things.

Quixel bridge is bugged in Unreal 5 beta for Aximmetry, you can fix it like it is described here: https://my.aximmetry.com/post/2223-unable-to-install-quixel-bridge-plugin-i If you did not do so before, then this will probably fix the missing material issue.

Otherwise, there are several threads on Quixel about this issue with Missing Material:
https://help.quixel.com/hc/en-us/community/posts/360014093177-Material-Missing-Function
https://help.quixel.com/hc/en-us/community/posts/360014444717-Material-Texture-Missing-in-Unreal-SM5-Missing-Material-Function-

Warmest regards,