Blockchain.com gives each user their own wallet (with secret key). If you are sending something to it, you don't need to specify the memo. Or you can put any text inside if you want.
On Coinbase all users share the same wallet and if you make deposits to it you need to specify a memo in the transaction so that Coinbase can differentiate you. If you click on the receive XLM link, Coinbase will display you the address and memo you need to use (example).
In your case, you will need to contact Coinbase support. The XLM arrived in Coinbase's wallet, they just can't associate this transaction with you.