Unable to add day and date to my Apple 5 watch face.

The most likely reason you're having this problem is that you deleted the default calendar app from your phone. Reinstall that and you should have access to the complications again.

Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

Depending on the specific face you selected, you may or may not be able to add specific complications.

Change the watch face on your Apple Watch - Apple Support

So you got it working? Great!