Please be sure you have two-factor authentication enabled. Apple requires that iCloud accounts have two-factor authentication enabled in order to use apps such as Fantastical. Please see our iCloud page for more info. This is required by Apple's servers to increase security, this is not something we can disable.
Have a question? Check the Frequently Asked Questions as it may already be answered. If it's not, please don't hesitate to contact us.
If you'd like to send us a screenshot to illustrate your problem, submit your message and you'll receive an email confirmation. You can then reply to that email and attach screenshots.
Please include the version of macOS or iOS you are using as well as the version of the app you're asking about.