You cannot use a 7.x version of Pega Customer Service with Pega Platform 8.x.

Pega Platform 8.1 introduced a new set of role-based portals (App Studio, Admin Studio, Dev Studio, and Prediction Studio) for the configuration of applications that are built on Pega Platform. The change was significant and requires Pega Customer Service and Pega Platform be on the same major version for compatibility purposes. This information is included in the Pega Customer Service 8.1 Release Notes on the Pega Customer Service product page.