Hi @alps, thanks for your message.
When using Sync there are actually two paths you can specify. One of those is the one you already set, that is the one where the local realm is saved.
There is also AppConfiguration.BaseFilePath that is used for storing all local data for your application, including metadata for users and synchronized Realms. The error you are getting is because probably the default folder for this parameter cannot be used. This can be set when when creating the App. For instance:
var config = new AppConfiguration("appId")
{
BaseFilePath= --- your path ----
};
var app = App.Create(config);
Setting also this to an accessible folder should solve your issue.