One question that springs to mind is in which context are you deploying this (User or SYSTEM)?
If as the user - Do your users have permission to access the share ($Source)?
If as SYSTEM - The System account will not likely have access to the share ($Source) - Unless your folder permissions are very open (“Weak security”), also if it is run as System the $User and $Destination variables will not contain valid paths
I would suggest changing your $Destination line to:
$Destination = "C:\users\$($User)\CATIA_V5_DMU"
As this would likely cause less errors in the path due to unusual characters contained within the Username
and amend your Copy line (-Recur becomes -Recurse):
Assuming this then works correctly, you will have just copied the entirety of the $Source to the $Destination, So the certutil command could then be modified to install the prod-bundle.p7b from the local copy in the $Destination folder, this way there should be no risk of a network interruption preventing this cert being installed.
You could even check it exists before attempting to install it
e.g.