In Magento vendor folder below file path
* This message display as 2 part
1) "There was a problem with the subscription:" In execute the function.
2) "Sorry, but the administrator denied subscription for guests. Please register." In same file "validateGuestSubscription" protected function throw message.
Your Message comes from NewAction.php file,
Check file path in github,
In above statement, $e->getMessage() comes dynamically from same file at below location,
If you want to change only string you can directly change from en_US files. Just create en_US.csv file into your code or theme folder,
Please let me know if you have any issue.