Calendar syncing - installing an onEventUpdated trigger

Topline: I'm having a problem I don't understand; I can run this code if I do so manually in the script editor:

ScriptApp.newTrigger('functionname').forUserCalendar('usercalendar').onEventUpdated().create()

But, when I install the application from manifest and attempt to run the same code from the users perspective, I get an error "Exception: We're sorry, a server error occurred. Please wait a bit and try again. [line: 15, function: installTrigger, file: Code]"

Context: I'm working with a client to set up a conferencing extension for Google Calendar.

Part of that involves keeping their server roughly up to date with any calendar changes from the user - if I've got a conference meeting at 10am, but at the last minute I change it to 11am, the server needs to know!

The documentation actually has a section on this: https://developers.google.com/gsuite/add-ons/calendar/conferencing/sync-calendar-changes . The idea is the add-on installs a trigger that runs whenever an event gets updated. You need to do some filtering to make sure you're only sending relevant data, but that's the broad mechanism. I simply can't replicate the ability to create a calendar onEventUpdated trigger!

To demonstrate the issue, I've cut down the code to bare bones below. This code makes a sidebar available in google calendar with a button. When you click the button, it should install an onEventUpdated trigger on the calendar you've specified in the code.

Code.gs:

//Attempting to install an onEventUpdated - it appears I can't use 'forUserCalendar' when installing as a user, but can when I manually select 'installTrigger()' function in the code editor?

//Functionality/scopes based on those demonstrated here:
//https://developers.google.com/gsuite/add-ons/calendar/conferencing/sync-calendar-changes
//https://developers.google.com/gsuite/add-ons/calendar/conferencing/conferencing-sample?hl=fr#add-on-manifest

function onHomePageOpen(e) {
    let newButton = CardService.newTextButton().setText('Try install sync').setOnClickAction(CardService.newAction().setFunctionName('installTrigger'))
    let card = CardService.newCardBuilder()
        .setHeader(CardService.newCardHeader()
            .setTitle("<h1>Test install sync</h1>"))
        .addSection(CardService.newCardSection().addWidget(newButton))
        .build()
    return card
}

function installTrigger(e) {
    var triggerBuilder = ScriptApp.newTrigger('syncupdate');
    var triggerBuilder2 = triggerBuilder.forUserCalendar('<<YOUR EMAIL IN HERE>>').onEventUpdated().create()
    console.log('triggerBuilder2', triggerBuilder2)
}

function syncupdate() {
    console.log('triggered on update of an event!');
}

manifest.json

{
  "timeZone": "Pacific/Auckland",
  "dependencies": {},
  "oauthScopes": [
    "https://www.googleapis.com/auth/calendar",
    "https://www.googleapis.com/auth/calendar.readonly",
    "https://www.googleapis.com/auth/calendar.events",
    "https://www.googleapis.com/auth/calendar.events.readonly",
    "https://www.google.com/calendar/feeds",
    "https://www.googleapis.com/auth/script.locale",
    "https://www.googleapis.com/auth/calendar.addons.current.event.read",
    "https://www.googleapis.com/auth/calendar.addons.execute",
    "https://www.googleapis.com/auth/calendar.addons.current.event.write",
    "https://www.googleapis.com/auth/script.scriptapp"
  ],
  "exceptionLogging": "STACKDRIVER",
  "runtimeVersion": "V8",
  "addOns": {
    "common": {
      "name": "Test Install Sync",
      "logoUrl": "https://www.gstatic.com/companion/icon_assets/tasks2_2x.png",
      "layoutProperties": {
        "primaryColor": "#123763",
        "secondaryColor": "#1a73e8"
      },
      "useLocaleFromApp": true
    },
    "calendar": {
      "homepageTrigger": {
        "runFunction": "onHomePageOpen"
      },
      "currentEventAccess": "READ_WRITE"
    }
  }
}