301 Redirect Not WOrking

Hi, I hope everyone is having a nice day. 

I have been trying to send a redirect from my page to our payment processor.  

So I've put this in the URL mapping; 

/paymongo888 -> www.paymongo.page/l/thebigbrainbrand888 301

When I go to the web address https://brainsandbeautyph.com/paymongo888 the page I get this message
 

We couldn't find the page you were looking for. This is either because:

• There is an error in the URL entered into your web browser. Please check the URL and try again.
• The page you are looking for has been moved or deleted.

You can return to our homepage by clicking here, or you can try searching for the content you are seeking by clicking here.

and the browser address bar displays this;

https://brainsandbeautyph.com/www.paymongo.page/l/thebigbrainbrand888

I'm sure I'm missing something.  I appreciate the help/feedback.  

Thank you and have a nice day.