It looks like your references are broken. A reference is something that tells Unreal Engine where to find an asset, therefor even though the assets exists the engine would not know were to find them. This issue is common when moving your files between projects.
You can try to figure out which reference is missing/broken and try to find it in windows explorer, but that could be tedious to do.
Just open each material and specify manually the missing texture, then go to each mesh and specify manally each material.
And don’t forget to save!
I hope this helps 