You do not have permission to call showSidebar

To allow this add the next scope to appsscript.json

{
  "oauthScopes": [
    "https://www.googleapis.com/auth/script.container.ui"
  ]
}

[VAggrippino]

Never mind… The problem is onOpen(). I apparently can’t show any kind of Ui without user interaction: https://issuetracker.google.com/issues/69238694#comment7

[VAggrippino]

Sure. I appreciate you taking a look.

It works if I run the function from the script editor, but not if I run it from the onOpen() handler.

Here’s the error message in the StackDriver Logging:

Exception: You do not have permission to call Ui.showSidebar. Required permissions: https://www.googleapis.com/auth/script.container.ui
   at onOpen(Code:5:8)

Here’s some very basic sample code recreating the problem:

function onOpen() {
    const html = HtmlService.createHtmlOutputFromFile('Index');
    const ui = SpreadsheetApp.getUi();
    ui.showSidebar(html);
}

and my appsscript.json:

{
  "timeZone": "Asia/Hong_Kong",
  "dependencies": {
  },
  "exceptionLogging": "STACKDRIVER",
  "runtimeVersion": "V8",
  "oauthScopes": [
    "https://www.googleapis.com/auth/script.container.ui"
  ]
}

And the spreadsheet this is bound to is here