Welcome to the Let's Encrypt Community! 
The usual reason for a certificate not being acquired by cert-manager is lack of application of an ingress manifest specifying the host for the certificate. I do see that step though in the tutorial you linked. Please post the log from cert-manager. It will greatly aid with debugging.
In my corporate experience with cert-manager I have never needed to create or apply a yaml file for the certificate itself since cert-manager generates and populates the k8s secret containing the certificate. I have used ingress-nginx there though.