In Mac system preferences > iCloud the box of reminders must be checked see this article https://support.apple.com/en-in/guide/reminders/remnd5faf3ad/mac
If you keep your reminders in Internet accounts — for example, iCloud, Exchange, Yahoo or AOL — you can use the accounts in Reminders on your Mac, and access all your reminders in one place. Set up your iOS devices to use the same account, so that your reminders stay up to date across all your devices.
Means both iPhone and Mac must be signed in with same Apple ID and password with high network speed , the apple devices must be upgraded and updated with latest version see this article also https://support.apple.com/en-us/HT203521