use a script task instead of a file system task;

the code would look something like this:

string WorkingDirectory = (string)Dts.Variables["WorkingDirectory"].Value;
      string ArchiveDirectory = (string)Dts.Variables["ArchiveDirectory"].Value;

how are you sending the data to CSV in your job step? sqlcmd? bcp? powershell? SSIS? both sqlcmd and bcp can add those delimiters , and force column widths .

ok i was able to duplicate the error and a resolution. here's the tiny differences.
I created am XLSX document , with a default sheet name of Sheet1.
i used...

yes, with a calculated, persisted column, that uses the real identity column. 
Note this is just creating a formatted version of your identity, so it's a little repetitive.
You might...

the error is most likely inside your function.

just based on the name, it's probably trying to find the "latest" invoice, but what if...

my comments got lost somehow.
the issue the OP is encountering is the Service Account that the SQL Agent runs under does not have access to  "C:\Users\myName\Desktop\WUpdates\"
that path is a private...

definitely, use something that will create a proper filename, and handle ending slashes

the below returns "C:\NewFolder\20172018"
$filename = "20172018"
$file= "C:\NewFolder"+$filename
you mosdt likely need the file and...

the naming thing is caused by using the sp_rename command; there are a few questions of the day on the same issue.

when you use sp_rename on a view/procedure/function,...

can't you add a windows group a user,  grant it db_datareader then set the database to read only?
then you can add the 1000 users to the user group, and...

it's a two step process;
you have to associate the files at windows, and then tell SSMS that you want to edit those extensions as if they were sql code.
create...