It takes a few minutes for your iPhone to transfer the updated app to your Apple Watch after updating. Wait 5-10 minutes before trying to use Fantastical on your Apple Watch after updating. If Fantastical still doesn't open after waiting 5-10 minutes, restart your Apple Watch and try again.
If Fantastical says it doesn't have access on your Apple Watch, please try the following:
Fantastical can only show reminders that will be due soon when you are not near your iPhone. Reminders that are due soon will be shown, but you will not be able to see reminders without a due date or reminders that are due in the past or far in the future.
Apple Watch only allows a small amount of data to be sent from your iPhone to your watch. Because of this, Fantastical on Apple Watch may not be able to show events too far into the future and instead keeps you informed of events that are more likely to be relevant to you.
This is due to a bug in watchOS.
To fix this, launch the Fantastical watch app and then hold the watch's side button until the 'Power off' option appears. When this appears, hold down the digital crown for a few seconds to force close the app.
After re-launching Fantastical the complication should work as expected.