With Windows scripting, it always helps to mention the exact version of Windows and whether you have installed any extended Windows scripting products and whether you have any programming languages installed.
The issue is that you are attempting to read the same file as you are writing. You can go ahead and use the code you wrote but instead of writing to the same file, write to a temporary file and then copy it over the original file
for /F "delims=" %%j in (list.txt) do echo.STARTTEXT\%%j >> tempfile.txt
copy /Y tempfile.txt list.txt