As @Adonalsium mentions in the comments, Data Valet manages the Starbucks public WiFi. They're trying to redirect you to the "agree to terms" page to login to the WiFi. Go to a page in a web browser that is not protected by ssl (neverssl.com is great for this), and it will redirect you to the portal page so you can agree, and then this will go away.
The error is the security on your system working as intended, by blocking the connection to a server that isn't the one you were trying to connect to. When it asks something like "Do you want to proceed?", answer "No" or whichever answer will cancel the connection, to avoid sending secrets like authorization tokens to an untrusted server. Login to the portal as mentioned above, and then try again.
The name for the mechanism that redirects you to the terms page and doesn't let you go anywhere else until you agree is Captive Portal. Captive Portals are a common and annoying pattern for "protecting" public WiFi access points, but they have the annoying fault that they essentially need to conduct an attack against your connection to work.