You may have pending WordPress subscriptions already, in which case they won't let you sign up for new stuff until you confirm or delete the pending ones. How to do that??
1. Visit https://subscribe.wordpress.com/.
2. Enter your email address and click Request Details.
3. After requesting your details, you'll be sent an email with a link in it. WATCH FOR IT! By clicking the link, you'll be taken to a page where you can manage their subscriptions.