Save multiple data into database from distance relationships in laravel

I am trying to save multiple data which is having a many-to-many relationship. My store method is like the below.

public function store(Specification $specification, Fraction $fraction, Request $request)
{
    $data =  return request()->validate([
        'specification_no' => 'required|string',
        'grade' => 'required|integer',
        'gname' => 'required|integer',
        'gsize' => 'required|integer',
        'fractions.*.fraction_min' => 'required',
        'fractions.*.fraction_max' => 'required',
        'fractions.*.unit_id' => 'required',
        'fraction_sieves.*.sieve_id' => 'required',
        'params.*.para_min' => 'required',
        'params.*.para_max' => 'required',
        'units.*.unit_id' => 'required',
    ]);

    // Save specificatio without duplicates.
    $specification = new Specification($this->specificationValidation());

    $checkSpecification = Specification::where('specification_no', request()->specification_no)->first();

    if (!$checkSpecification) {
        $specification->save();
        $specification->grades()->syncWithoutDetaching($data['grade']);
        $specification->gnames()->syncWithoutDetaching($data['gname']);
        $specification->gsizes()->syncWithoutDetaching($data['gsize']);
        $specification->sieves()->syncWithoutDetaching($data['fraction_sieves']);

        // $specification->load('sieves.fractions');

        $fraction = $specification->sieves()->fractions()->createMany($data['fractions']);
        dd($fraction);
    }        
    return redirect()->back()->with('alert', 'Duplicate Specification number.');
}

Models & migrations like below.

Fraction-Model

class Fraction extends Model{
protected $guarded = [];

public function sieves()
   {
       return $this->belongsToMany(Sieve::class);
   }
}

migration - fraction

    public function up(){
        Schema::create('fractions', function (Blueprint $table) {
            $table->bigIncrements('id');
            $table->double('fraction_min');
            $table->double('fraction_max');
            $table->unsignedBigInteger('unit_id');
            $table->timestamps();
        });
    }

model - sieve

class Sieve extends Model{
    protected $guarded = [];
    public function specifications(){
        return $this->belongsToMany(Specification::class);
    }

    public function fractions(){
        return $this->belongsToMany(Fraction::class);
    }
}

migration pivot table - fraction_sieve

    public function up()
{
    Schema::create('fraction_sieve', function (Blueprint $table) {
        $table->bigIncrements('id');
        $table->unsignedBigInteger('sieve_id');
        $table->unsignedBigInteger('fraction_id');
        $table->timestamps();
    });
}
public function down()
{
    Schema::dropIfExists('fraction_sieve');
}

When I use the below code

$fraction = $specification->sieves()->fractions()->createMany($data['fractions']);

to save fractions to fractions table with distance relationships from specifications->sieves->fractions, I am getting errors like this.

BadMethodCallException Call to undefined method Illuminate\Database\Eloquent\Relations\BelongsToMany::fractions()

Normally this could be the way of creating new multiple data into db. But I can not understand why I'm getting errors. Can someone explain what I've messed up?