code: '5008', message: 'User has no permission to subscribe'

This error occurs when I try to subscribe to the channel

That means, the logged in user doesn’t have permissions to subscribe for a channel

Please read the following documentation topics :

    [url=https://backendless.com/documentation/users/rest/users_user_roles.htm]Security and Roles[/url] [url=https://backendless.com/documentation/users/rest/users_global_permissions.htm]Global Permissions[/url] [url=https://backendless.com/documentation/users/rest/users_asset_container_permissions.htm]Asset Container Permissions[/url] [url=https://backendless.com/documentation/users/rest/users_asset_permissions.htm]Asset Permissions[/url]

The problem is that I try to subscribe after success sign in(AuthenticatedUser can subscribe)

Are there any other roles restrictions in your app? If at least one role denies access, then the access will be denied.