Can't buy license on chrome, told to sign in despite being signed in

I've tried updating the browser, signing out and back in, etc. Can't seem to get through the license purchase flow. Just keep seeing this message:

Warning: Chrome is not Signed In.

Chrome Sign In required to validate your license key and to configure online backup on your Google Drive account. Please Sign In to Chrome and try again.

Alternatively you can open Sign In to Chrome dialog from Chrome settings, or by clicking the Profile name on the top right of any normal Chrome window (above the tabs strip).