How do I cancel mlb subscription when no tab is on phone?

If you don’t see the option to unsubscribe, make sure you’re logged in using the correct Apple ID. It’s also possible that you already successfully canceled the subscription.

How to unsubscribe, change subscription, or manage your subscriptions:

https://support.apple.com/en-us/HT202039

See the instructions here. You can do it on the computer. View, change, or cancel your subscriptions - Apple Support