Is this a normal image in the background or something else?
Do as it suggestests, UE4 on mobile runs on OpenGL ES which a lot more restrictive and might require power of two texture sizes. Optimally textures should have power of two sizes (2 ,4, 8, 16, 32, 64,128, 256, 512 and so on and so on on either axis), some older GPU require it for mip-mapping and other features to work, i know WebGL (which follows ES specification) had exact same limitation.
So either resize the texture to power of two size or look on settings as it hints
Thank you so much!
You need to make sure that mobile devices only support textures and images with formats like: 256x256, 512x512, 1024x1024… .
Your background image does not match these conditions.
Try to scale the background image and it should work:)