There are plenty of videos on youtube explaining how to UV Unwrap things in 3DSMax. Its a pretty big area of 3d artwork after all. Main thing is you need to make sure that no polygons are overlapping and they stay within the 0…1 space but maybe its better if you don’t know how to do that to use the automatic unwrapper in Unreal. Click on the mesh in the content browser, go to “window->generate UV’s” and it should open a new tabbed window in the mesh window. Select “create new” option and make sure UV channel to save results to: is set to UV Channel 1 and click apply.
This automatically unwraps the mesh as you would have done it in 3dsmax (it doesn’t work well for complex meshes though). But should at least stop you having errors.