Go to the following section of the 2.7 Creating the Developer Portal guide:
https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.7/html-single/creating_the_developer_portal/index#liquids

Click on the Liquid Reference link:

To learn the basics about Liquids, see the Liquid reference.

Which returns a 404. The correct location of the Liquid Reference is:
https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.7/html-single/liquid_reference/index