The reason this is failing is likely due to permissions given the fact that the Client service runs as LOCAL SYSTEM.
See Copy file from network share to client Downloads folder and Copy the file from one location to network drive for more context, as well as some suggested approaches.
Ultimately, in such cases, it’s usually best to have the installation packages available on a web server (including for instance the Root Server itself), and downloaded via prefetch.
You might also consider leveraging the Software Distribution capabilities of BigFix to simplify the workflow and management, as well as provide a lot of useful features: https://www.ibm.com/developerworks/community/wikis/home#!/wiki/Tivoli%20Endpoint%20Manager/page/Software%20Distribution