You may want to use an SSH client on the OCI server to attempt to ssh to the switch in question using the credentials you've configured in OCI. This will help you confirm that SSH connectivity is possible and that your credential is correct.
Also keep in mind that if you have chassis discovery turned on, OCI will attempt to connect to each director-class switch in the fabric. So if your credential is configured on one switch but missing from the next, that next switch may be the one throwing this error. You will be able to see this by looking closely at the error message, perhaps in the logs.