The order could not be processed with the reference code [xxx], PayU Laravel

I am trying to get my signature for payu webcheckout implementation I think the error is in the signature field, I have the api key defined in the controller

<form method="post" action="https://sandbox.checkout.payulatam.com/ppp-web-gateway-payu/">
  <input name="merchantId" type="text" value="508029">
  <input name="accountId" type="text" value="512321">
  <input name="description" type="text" value="{{ $plans->description }}">
  <input name="referenceCode" type="text" value="{{ $plans->id }}">
  <input name="amount" type="text" value="{{ $plans->price }}">
  <input name="tax" type="text" value="0">
  <input name="taxReturnBase" type="text" value="0">
  <input name="currency" type="text" value="COP">
  <input name="signature" type="text" value="md5($apy_key." ~ "."508029 "."~ ".$plans->id."~ ".$plans->price."~ "."COP ")">
  <input name="test" type="text" value="1">
  <input name="buyerEmail" type="text" value="test@test.com">
  <input name="responseUrl" type="text" value="https://poligonourbano.soluttolabs.com/">
  <input name="confirmationUrl" type="text" value="http:// www.test.com/confirmation">
  <input name="Submit" type="submit" value="Enviar">
</form>

Controller function:

public function planView() {
  $apy_key = "4Vj8eK4rloUd272L48hsrarnUA";
  $plans = Plan::findOrFail(2);

  return view('plans', compact('plans', 'apy_key'));
}

The problem is indeed coming from the way you are creating the signature

when you put md5($apy_key."~"."508029"."~".$plans->id."~".$plans->price."~"."COP") in "" it is considered as a string you should wrap the md5 with {{ }}.

Please try to create the signature in the controller and pass it as a value to the view, it is not a best practice to handle any logic in the view.