Use Username:admin Password:pass in your secrets file when setting up the mongo-secret.yaml file.

Open a terminal and do: echo -n admin | base64

and

echo -n pass | base 64

Copy the resulting encoded username and password into your secrets.yaml file and then use admin and pass as your arguments for username and password when the prompts comes up.

That works for me.

It is also discussed here: mongo-express service in minikube doesnt seem to work with its username and password