This happens when your local system time is out of sync with the network time. If you force a re-sync, you will be able to log back into the site and then successfully complete the lab or challenge. Here are the steps to force a re-sync of your local system time:

Windows 10

1. Click on clock and select “Adjust date/time”.
2. Toggle on/off the "Set Time Automatically"

macOS

1. Open up your local terminal application
2. Execute this command:

sudo ntpdate -u time.apple.com

Good luck!