Apple Music on iMAC

(1) Have you already set up a subscription to Apple Music? Because that is where Apple Music shows items that are not in your iTunes Library. If no, then you first have to sign up for Apple Music, then after a few setup screens, those two tabs should populate.

(2) If you have already set up an AM subscription - like on another Apple Device like an iPhone - then is the iMac signed into the same Apple ID because that is what the AM subscriptions are tied to.

If you already have the subscription set up on the iMac, then perhaps quitting the Music app and restarting the iMac could work. Otherwise, you will have to sign out of your Apple ID for at least the Music app and then sign back in - however this will cause Apple Music to start the entire process of reading and uploading your music library again unfortunately.