You can use plan to show the next calendar event with Sketchybar
If you intend to use the plan watch feature to monitor the ~/Library/Calendars directory to fire Sketchybar events, you need to give /opt/homebrew/bin/plan "Full Disk access".
macOS protects files in ~/Library/ from potential malicious access.
You can give plan access, by opening "System Settings > Privacy & Security > Full Disk Access", Click on the "+" icon, and navigate to /opt/homebrew/bin/plan (Cmd+Shift+G in the Finder lets you navigate more directly by typing)
This is different from the access to the actual "Calendar" app. macOS will actually prompt you for that.