Hiya, I'm buying some XMR from Changelly (exchanging for my BTC) and I am having some trouble. I understand the payment ID is optional, but I want to do it. My wallet I am using is the Monero Core Wallet from the website, the GUI version.

I already copied the top "Address" for my receiver's address field, and when I generate the "Payment ID" and copy it into Changelly, it says it is invalid. What am I doing wrong?

What consequences are there if I were to buy XMR from Changelly with just my XMR address without the payment ID? Will the XMR get lost or not deliver and I lose my BTC?