So I made a simple door in blender and textured it with the painting tool in blender, I exported it as a .fbx and imported it into unreal engine, the mesh shows up but it doesnt have the texture from blender… how do I fix this? do I have to do something else?
whatever method you create textures you need to save to an image file–like .png or .tga and make sure their pixel dimensions are values that are a power of two.
this still doesn’t really answer my question tho 
why do I need to save it as a .png or .tga? do I save the texture as a .png and put that into unreal engine too? what do I do?
If you did not manage to export the texture from Blender you can always do it manually. Save that image from Blender (Image -> Save as image if I remember correctly) as PNG (for example). Then copy and paste it in file explorer to your UnrealProject/Content folder. Open up the material of the door in UE and plug the texture in. Sorry I don’t have access to blender or UE at the moment to check the actual step by step process.