To access content on this site, you need to sign in with an approved partner domain account. Your contact at Google creates a partner domain for you, but you need to accept and configure your account. See Partner Domain Accounts.
Once you have access to your partner domain account, sign in with that account to access the content on this site.
Troubleshooting
If you're signed in but don't have content access or receive a 404 error when trying to access the documentation, confirm that you're signed in with your partner domain account and not another Google account. If you continue having trouble accessing content, send a message to your contact at Google.