Laravel Array to string conversion not working

I want to update a totcosty field in the User table but it is throwing this error everytime and it is not updating the field

this is the function for execution:

 public static function cost(){}

$user_id = auth()->user()->id;
$user = User::find($user_id);
$total = Helper::totcost();
   //dd($tot_amt);
$user->totcosty = $total;
$user->save(); 
}

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