This is because you have already one version installed in the local machine and Informatica will not allow you to install 2 in same machine. i hope this is windows machine where you are trying to install.
How exactly did you uninstall?
What Windows version?
What's the repository DB and version?
First you need to unset the INFA_HOME,INFA_DOMAIN_FILE from the cmd line to remove the registry for existing informatica version and disabled the existing Informatica 9.6 service from services.msc. then again try to install it.