Summary: There is an issue affecting some Macs running OSX 10.13.4 that can prevent sensors/peripherals from connecting via native Bluetooth. This isn't an issue with Zwift, but the Mac operating system itself.
Workaround: Some users on the Mac community forums have found a workaround that involves deleting plist files.
- From the Finder window, press Command + Shift+ G to call the “Go to Folder” function, then enter the following: /library/preferences/
- Find and remove the “com.apple.bluetooth.plist” and “com.apple.bluetooth.plist.lockfile” files, if available.
- Reboot your system (necessary for the fix to work).
Alternatively, if you aren't comfortable modifying your system files, you can use the Zwift Companion app to pair your BLE sensors instead.
As this is an Apple issue, we don't know if or when a fix will come out, so further questions should be directed to Apple.