That path is invalid on the server.
See "creating file paths"
For server-side scripts, access is limited to the FileMaker Server Documents folder and temporary folder. See Paths in server-side scripts.
Set the path to a variable, like $path, then above that script step, set a variable with a valid path, like this:
Set Variable [ $path ; "file:" & Get(DocumentsPath) & "COA/" & $FileName ]
I had just found that link and changed my file path too
filewin:/C:/Program Files/FileMaker/FileMaker Server/Data/Documents/Reports/$FileName
and it worked perfect