The simple answer is that Binance Smart Chain uses ETH based addresses, so Changelly, or any other source one tries to use, will reject it.
First, one needs a Binance Wallet, and a “bridge” to send things from the Binance Chain, to the Binance Smart Chain.
What I did was buy the BNB, and send it to my Binance Smart Chain Wallet Extension, making sure the address starts with bnb.
From the Extension, I then send what I want from the Binance Chain Wallet, to the Binance Smart Chain Wallet, all in the same extension. This address starts with a 0, like Ether addresses.
Now that I have what I want in the BSC, I can then send that to my MetaMask BSC.
With having to do those extra steps, and, though currently minuscule, I don’t see the practical reason to use MM for BSC. The only practical reason, really, is if you’re trying to use something–and there are a few yield farms–that only allows you to connect MM to the BSC. Otherwise, if given the option, simply using the BSCW, and doing one less step, and one less fee, is better, imo. There are some that just love MM, but to me, that’s not practical, but each to their own. It’s at least good to know, when there may be a time when MM is the only option.