Unable to package Zendesk app(zcli)

When I run "zcli apps:package" in the command prompt in the root directory of the app, the following error is displayed.

»   Error: AxiosError: Request failed with status code 500

Although an error occurred, a zip file was created in the tmp folder, 
and when I tried to upload the app using "zendesk developers", the following error was displayed:

» The task failed because of an error: invalid byte sequence in UTF-8

» 1 error must be addressed before proceeding
  Error: Something went wrong when trying to validate your app. Please try again or contact Zendesk Support:
 

I'll also post the manifest file.

{
 "name": "***",
 "author": {
   "name": "***",
   "email": "***",
   "url": "***"
 },
 "defaultLocale": "en",
 "private": false,
 "location": {
   "support": {
     "ticket_sidebar": {
       "url": "assets/HTML/iframe.html",
       "flexible": true
     }
   }
 },
 "version": "1.0.1",
 "frameworkVersion": "2.0"
}