Accidently sent bitcoin from blockchain wallet to coinbase, but used a bitcoin cash address for the transfer, by mistake. Can I fix this so I can get my bitcoin? On my coinbase wallet, i don't see the bitcoins anywhere.
It sounds like you sent your bitcoin to an address that you do not own, and is not associated with your Coinbase account. If this is the case, it's important to understand what has happened. Forgive me if this is review for you, but others may find it useful.
Bitcoin and Bitcoin Cash addresses are the same in structure. These addresses are a hashed and checksummed representation of the public key of a asymmetric key pair. In order to spend them, you need the private key. If you have the private key, you control any money paid to the address on either blockchain.
It appears as though you copied a Bitcoin Cash address generated by Coinbase, and sent Bitcoin to it thinking it was a Bitcoin address tied to your Coinbase account. I'm not sure how this happened, but the result is that your Coinbase Bitcoin account knows nothing about the transaction. You transferred your Bitcoin to an address that Coinbase may not even realize exists on the Bitcoin blockchain.
Here's the problem: You don't have the private key, only Coinbase does. Coinbase may only recognize this private key as a key to a Bitcoin Cash address, but it is also a key to a Bitcoin address. You will need to get them to use this private key to generate a Bitcoin address that is tied to your account. You cannot do this without their cooperation. You don't own the private key, and therefore, you technically don't own the Bitcoin.
As a general rule, do not consider any Bitcoin held by a third party to be your Bitcoin. It's not. Bitcoin is owned by whoever controls the private key. In the case of a Coinbase wallet, Coinbase owns the Bitcoin on your behalf. They are nice enough to give you a web interface to let you use their Bitcoin that you paid for.