This continuous message prompting you to sign in to your WordPress plugin could be cause by one of these two issues.
1. You are signed in as a guest user in your plugin. In this case, you need to sign up with MailMunch or use sign in option to login with your existing MailMunch credentials.
2. You have multiple WordPress plugins installed by MailMunch. In this case, you need to remove the plugin that is not in use and only keep one plugin installed.