Hi @deanima,
The 403 Forbidden error is an HTTP status code that means that accessing the page or resource you were trying to reach is absolutely forbidden for some reason.
MailPoet 3 requires write permissions to create content inside the WordPress’s /wp-content/uploads folder. MailPoet uses this folder to store cache and other transient files.
Please ensure that /wp-content/uploads folder permissions are set to “755”, “775” or “777”. You can do that via FTP.
Let us know.