Cannot install or uninstall the Citrix Receiver or Web Plugin or and installing message appears when logging into the website
If an older Citrix Receiver (aka Online/Web Plug-in, PNAgent, ICA Client, ICAWebWrapper) was installed on a computer then the latest Citrix Receiver may not install properly because it cannot overwrite or upgrade the older installation. In this case the older client must be removed manually. See here for how to.
After following the instructions in the above article, install all Windows updates and reboot the machine before installing the latest Receiver from citrix.com.