Problem on customer checkout

I was recently told by a customer that as he was checking out he received a message that says “There appears to be a problem subscribing you to our newsletter/s. Please let us know to manually add you ourselves. Thank you.” He sent me a screen shot but I don’t know how to post an image here. He said it wont let him checkout and purchase the item. Can anyone tell me what this means and whats causing it?

https://wordpress.org/plugins/woocommerce/