First thing you need to focus on Laravel will look for pivot table name based on the alphabet order ..
It will look for category_subject but you have created subject_category and in the relationship there you need to tell the Laravel that which table and which column manually..
Your relationship is many to many for subjects and category👍
Here is the code try this.
public function categories()
{
return $this->belongsToMany(SubjectCategory::class, 'subject_category', 'subject_category_id');
}