It’s important to understand how Coinbase handles digital currency transactions to ensure any refunds reach the sender’s account. Coinbase does not support digital currency being returned directly to an address it was sent “from” (in the digital currency world, we call these input addresses).
When a Coinbase user sends digital currency from their wallet, the address it comes "from" is one of Coinbase's many hot wallet addresses. Any coins sent back to that address would be sent to Coinbase, not your own wallet.
For refunds sent back to a Coinbase customer, there are several ways you can go about obtaining an address which will accept refunds for a particular transaction.
Once you have a digital currency or email address provided for refund, use the send page, include the address provided, enter the amount to be refunded, and click 'Send Funds'.
If the party you are trying to refund is not using Coinbase, you will need to reach out to them directly in order to obtain an address that can accept refunds.
More information on Coinbase transactions is included in the following article:
Why can't I see my transaction in the blockchain?