Sometimes you may receive an error when using Google services after establishing a connection to the VPN. There are few possible reasons due to which you can get such error.
In some cases, you may receive an error "Our system has detected unusual traffic from your computer network".
When connected to VPN we use shared IP space and hundreds of people can be connected through a single IP address at one time.
The error page most likely shows a reCAPTCHA. To continue using Google services, solve the reCAPTCHA. It's how Google knows you're a human, not a robot. After you answer the reCAPTCHA, the message will go away, and you can use Google services again.