you need to extract the textures and materials if you using fbx in unity, you can go to the inspector, on materials tab , extract textures and materials , a window will pop up and you just need to click fix now :) that's all have a nice day.
'(if you don't see the image right click and open it in a new tab)
if you have more questions i recommend to ask it in the discord server for faster anwsers : https://discord.gg/3wpTWpZZ
;)