I got two things to try.
First try creating a texture asset in the editor, then modifying it and saving it to see if that works.
Second UTextureRenderTarget is an abstract class meaning it has to be subclassed to be used as an UObject (I think?) might be wrong about that.
You can see the UMeta here https://github.com/EpicGames/UnrealEngine/blob/c3caf7b6bf12ae4c8e09b606f10a09776b4d1f38/Engine/Source/Runtime/Engine/Classes/Engine/TextureRenderTarget.h
Try using UTextureRenderTarget2D and see if that works for you, it’s not marked abstract