The error “The CSRF token could not be verified” usually appears when you submit a form that contains a file upload field and you select a file that is larger than the allowed file size for uploading on your server.
MailWizz has no control over this behavior, the upload file size is dictated by your web server and your PHP settings.
For nginx web server, have a look at this article.
For Apache web server with PHP, see this article.
For PHP itself, see this article.
Since this is a server side change, you might need to get in touch with your hosting to help you or hire a system administrator to do the changes for you.