The STAT command did not succeed. Error getting message number and sizes. Mail server pop.gmail.com responded: [SYS/PERM] Your account is not enabled for POP access. Please visit your Gmail settings page and enable your account for POP access.

The error message is clear: in order to retrieve email via POP3, you must enable it in your Gmail settings on their server at their web mail page. See their support article: https://support.google.com/mail/answer/7104828 Keep in mind that IMAP is usually preferred over POP3 for a number of reasons. This article discusses the difference between the two and why you might choose one over the other: https://support.mozilla.org/en-US/kb/difference-between-imap-and-pop3