微修正

app/Kintone/Models/PaymentPlan.php

@@ -68,7 +68,7 @@ class PaymentPlan extends KintoneModel
     protected function toArrayCustom(): array
     {
         return [
-            'payment_status' => $this->donePayment ? "支払済み" : "未払い",
+            'payment_status' => $this->donePayment() ? "支払済み" : "未払い",
         ];
     }