Wordpress Plugin on App Engine
Solution 1:
AppEngine Standard doesn't allow writing to local disk:
In the Google App Engine PHP 5 Standard Environment, the local filesystem that your application is deployed to is not writeable. This behavior ensures the security and scalability of your application.
https://cloud.google.com/appengine/docs/standard/php/googlestorage
If your application (or plugin) needs access to the local filesystem, AppEngine Standard is not the way to go. Nevertheless, AppEngine Flexible mode does, so you can try Flexible mode.