This is most likely caused by migrating engine assets from UE.
Using the first as an example - MA_Bark.uasset
It is referencing T_Default_BaseColor which is engine content from UE and is disallowed, you need to replace the reference to the texture from within the material instance or material graph. Same goes with T_DefaultMaskTexture in the same material.
Using this method go through the assets one by one and replace the engine content references and you will be good to go.