Using AppleScript to batch convert .pages to pdf
Replace the line of code producing the error with the following:

export front document to file (this_folder &amp; exportFileName) as PDF


There were two issues here:


export takes a document, so you need to pass it the document as opened by Pages.
You gave the file name, but not a containing folder to put the file with this name, so concatenate this with the containing folder path so Pages knows where to export to.