Hi gmg2000,
Welcome to the Apple Support Communities!
I understand that you have subscribed to iTunes Match from your Mac, but cannot turn it on from your iPhone. The setting is in Settings > Music, but I realize that you are not seeing the option there. The attached article below describes how to sign into iTunes Match on your iPhone.
iTunes Match on iPhone, iPad, or iPod touch - Apple Support
If you do not see the option as expected or as described in the article, can you let me know what options you do see? You can even post a screenshot if that is easier for you.
I also understand that you are receiving an alert on your Mac that says your subscription is set to expire. Is it possible that you signed up for the free subscription to Apple Music? If so, this warning is to let you know that iCloud Music Library would not be active without that subscription.
This will not be an issue for you; however, if you are subscribed to iTunes Match. iTunes Match uses iCloud Music Library as well. A more detailed description of iCloud Music Library and how it works with iTunes Match and Apple Music can be found in the next attached article.
iCloud Music Library: Understanding differences between Apple Music and iTunes Match - Apple Support
If you need assistance checking your subscriptions to verify what services you are subscribed to, please refer to the following article.
View, change, or cancel your subscriptions - Apple Support
Best regards