Your Message comes from NewAction.php file,
Check file path in github,
__('There was a problem with the subscription: %1', $e->getMessage())
In above statement, $e->getMessage() comes dynamically from same file at below location,
'Sorry, but the administrator denied subscription for guests. Please <a href="%1">register</a>.'
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,
en_US.csv#L81
string from en_US.csv
Please let me know if you have any issue.
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