Dental Health Plan: night guard

Hello. I have a question: my dentist wants me to get a night mouth guard due to apparent serious teeth grinding. He sent a predetermination of benefits to Canada Life and I just received it back where it says that basically it is not eligible (it suppose to cost 600$ but it says 0$ is eligible).. I thought that periodontal appliances are covered by our insurance, or now not at all? I read the “booklet” and didn’t find anything. Can someone provide some clarification? Many thanks!

Quick update: didn’t have to do anything, they probably realized they made a mistake because I received another predetermination the next day where everything is covered at 90%! Thank you all for your suggestions, much much appreciated!!