He invokes instead of Allah that which neither harms him nor benefits him. That is what is the extreme error.
The Saheeh International Quran Translation
يَدْعُوا۟ مِن دُونِ ٱللَّهِ مَا لَا يَضُرُّهُۥ وَمَا لَا يَنفَعُهُۥ ذَٰلِكَ هُوَ ٱلضَّلَٰلُ ٱلْبَعِيدُHe invokes instead of Allah that which neither harms him nor benefits him. That is what is the extreme error.