The message "Failed to download the application. Please sign in and try again. If the problem continues, please contact your system administrator" is observed on Mac OS and Windows users when attempting to install or launch Pulse Client version(s) 5.0R15+ and 5.1R8+ from their Web Browser. This may also occur after authenticating through the PCS / PPS from the Pulse Client and accepting the Upgrade prompt. This is because the PCS / PPC appliance is still being used to deliver the new Pulse Client. This type of Pulse Client upgrade process is also known as "Auto-Upgrade".
If you disable the "Enable web installation and automatic upgrade of Pulse Secure clients" option in your PCS / PPS appliance, then you will not observe the above message. This message will not be observed when you use the standalone application installer to install, launch, or upgrade to Pulse Client version(s) 5.0R15+, 5.1R8+, and above.
The "Enable web installation and automatic upgrade of Pulse Secure clients" setting can be found in the Administrator Console under Maintenance > System > Options.
NOTE: If you are upgrading to 5.2R1+ please see the article KB40362 - Unable to install, launch, or upgrade to Pulse Client version(s) 5.2R1+