You are updating data before removing the image so when you trying to remove image the path will be empty so that is causing error.

Check this code:

public function updateConcert(Request $request, Concert $concert)
    {
        $request->validate([
            'name' => 'required|max:30',
            'description' => 'required|max:200',
            'date' => 'required|date',
            'duration' => 'required|date_format:H:i:s',      
            'id' => 'required|numeric',
        ]);
    
        $concert = Concert::find($request->id);
        if ($request->hasFile('image')) {
            Storage::disk('public')->delete($concert->image);
            $imgName = microtime(true) . '.' . $request->file('image')->getClientOriginalExtension();
            $request->file('image')->storeAs('public/storage/img', $imgName);
            $concert->image = '/img/' . $imgName; // Cambiado para que sea idéntico al código de libros
            $concert->save();
        }
        $concert->update($request->input([specify here inputs needs to update]));
        $concert->artists()->sync($request->artists);
        return redirect('concerts')->with('success', 'Concert updated');
    }