Eddie,

I rewrote your code below;

Imports System
Imports System.Collections.Generic
Imports System.ComponentModel
Imports System.Data
Imports System.Data.SqlClient
Imports System.Text

Namespace WorkflowActivity.Scripting.Script
    '''<summary>
    '''Provides one or more methods that can be run when the workflow scripting activity is performed.
    '''</summary>
    Public Class Script1
        Inherits ScriptClass90
        '''<summary>
        '''This method is run when the activity is performed.
        '''</summary>
        Protected Overrides Sub Execute()
            Dim fPath As String = "D:\LF_Import\Credit Card CSV\" & Me.TokenReplace("%(FilePath)") & ".csv"
            Dim rowData As String = Me.TokenReplace("%(CSV Rows)")
            
            Using objWriter As New System.IO.StreamWriter(fPath, True)
                AppendData(objWriter, rowData)
            End Using
            
        End Sub
    
        Private Sub AppendData(byRef writer As System.IO.StreamWriter, byVal rowData As String)
            writer.WriteLine(rowData)
        End Sub
    
    End Class
End Namespace

I was not sure why you are calling the AppendData sub from within the Execute sub but I included that in the sample code to show you how it would be done.   Depending on the complexity of your code you could just as easily move the WriteLine method up into the Using block to write the data out to the file.

I would also suggest putting the StreamWriter in a Using block to properly dispose of it.  No need to call the Close method on it as it will be closed when the writer is disposed.