If you haven't receive any error, here are the troubleshooting steps we can perform to help you send it. Let's start by using an incognito window. From there, try to send the invoice to see if it'll work. If yes, we just need to clear the cache. There are times when you'll encounter an unusual behavior if the browser saves too much cache.
Also, let's try to use a different browser. This helps to determine if the browser you're currently using has issues. I'd suggest accessing your account through Google Chrome, Mozilla Firefox, or Microsoft Edge. Here's an article on how to record a customer payment.