How do I fix this?

Hello, I am new to unreal engine and I have a problem with shadows/textures. I have imported a map from a game called CS:GO using the correct plugin and I am trying to retexture the whole map; however, when I dragged the texture onto parts of the map this happened at the joins of the textures: Imgur: The magic of the Internet . I am unsure why this is happening, also another question is that when I place in my skylight nothing changes (lighting wise) so I am a bit unsure on how too light the scene.
Thanks in advance for the help.