Someone has already added that SSH key.

[Fatima Palacios]

want to add my ssh key and this message shows up

Someone has already added that SSH key.

[Nic Brough -Adaptavist-]

Welcome to the Atlassian Community!

This is what it says it is, you already added the key, but with a different user name (you should not do that, keys are not supposed to be shared between users).  You'll need to identify which account you shared it with and either use that account for access, or remove the key for the other account.

Or, generate a new key and use that for access, not sharing it with others.

If you don't know who you shared it with, you'll need to raise a support request with Atlassian to get them to identify the account.