Trying to add events to calendar does not work

Thanks for posting in the Community, @sylgour. 

At this time, adding new events to calendar is unavailable in QuickBooks Desktop for Mac 2019. As an alternative, I'd suggest using the To Do List feature so you can enter new events and set reminders for them. Let me guide you how. 

1. Go to Company from the top menu. 
2. Select To Do List. 
3. Click the Add (+) icon at the bottom of the To Do List window. 
4. Enter your specific events in the Note section. 
5. In the Remind me on field, set the date. 
6. Check the Show in Reminders box. 
7. Click OK. 

The screenshot below shows you the last five steps. 

Once you’ve completed a To Do event, mark this note as Done above the Remind me on field. 

I recommend checking out this user guide: Mac 2019. This PDF article provides you step-by-step instructions on how to effectively manage your income, expenses, lists, items, and your company file. 

I'm here anytime you have other concerns. Have a great day, @sylgour.