Please try the following steps one by one to see if that helps:
1) Rename OOBE Folder:
Mac OS: /User/<username>/Library/Application Support/Adobe/OOBE folder (Rename it to OOBE_OLD)
2) Download Creative cleaner tool http://www.adobe.com/support/contact/cscleanertool.html and remove Adobe files ( If you just have After effects, don't delete for any other files)
3) Then download Creative cloud app via https://creative.adobe.com/products/creative-cloud
Launch and start downloading After effects.