When I was in crypto for the first time I did the same thing. Since BSC & Ether both are different different blockchain that's why your money is transferred to the other blockchain. You sent to your Coinbase address and they are not supporting other blockchain of same token as you said. In this case you need to contact the the support via mail and explain all the things and give transaction related details to check the problem. Because you sent to an exchange which doesn't support the BEP20 token that is lost. But, if you request them then they might help you to refund the sent token.