Thanks @Marc Stridgen I dropped them a line and they said: for the error above ensure that the public key you are using is correct and matches the corresponding private key on the server. here https://my.wpengine.com/profile/ssh_keys on your User Portal, Click <Your name> at the top right, Select SSH Keys and Click New SSH Key we've got a documentation here with more details about it https://wpengine.com/support/ssh-gateway/
Ive asked why I need to create a SSH key when I haven't in the past and its been working for the last 4 years