Error: Object of class Illuminate\Database\Eloquent\Builder could not be converted to string in file

Question

so I'm trying to call this function with foreach array i'm pulling from supplier_id but i'm facing this problem

$request->validate([
            'destination_id' => 'required|exists:table_destinations,id',
            'supplier_id' => 'required|array',
            'supplier_id.*' => 'required|exists:table_suppliers,id',
            'account_id' => 'required|exists:table_accounts,id',
            'test_type' => 'required|string'
        ]);
        $destination_id = Destination::where('id', $request->destination_id)->first();
        $suppliers = Suppliers::whereIn('id', $request->supplier_id)->get();
        $account_manager = Account::where('account_id', $request->account_id)->first();
        $type_of_test = $request->test_type;
        foreach ($suppliers as $supplier) {
            (new classUtilities())->saveData($destination_id,  $supplier['id'],$account_manager, $type_of_test, $supplier['cost']);
        }

and that my classUtilities where i'm using my function

 function saveData(Destination $destination, $working_suppliers, Account $acccount_manager, string $test_type, $supplier_cost)
    {
        $working_supplier_id = Suppliers::where('id', $working_suppliers)->get();
        $working_suppliers_cost_rate = Suppliers::where('cost_rate', $supplier_cost)->get();
        $type_of_test = ItestTypeOfTest::where('test_name', $test_type);
        AllData::insert([
            'destination_id' => $destination->id,
            'working_supplier_id' => $working_supplier_id,
            'account_manager_id' => $acccount_manager->id,
            'type_of_test' => $type_of_test,
            'cost_rate' => $working_suppliers_cost_rate
        ]);
    }