I upgraded my iPhone 8 to IOS 13 and now and don't have my Reminders on Macbook pro 2012. What should I do?

See the following from Get ready for the new Reminders app - Apple Support

Upgraded reminders aren't compatible with earlier versions of iOS and macOS. If you upgrade your reminders on your iPhone with iOS 13, your iPad and Mac using the same iCloud account can’t access your reminders until iPadOS and macOS 10.15 Catalina are available. Learn what to expect until all your devices have the latest software.