The message tells you what to fix:
- "This doesn't look like a Steamworks wishlist file." Upload the file just as Steamworks gave it to you. If you opened and saved it in a spreadsheet app, download a fresh copy.
- "The file has rows for several games." Download the report for the one game you picked on the Wishlists page.
- "The file is larger than 2 MB" or it covers more than ten years. Download a shorter date range. Newer files simply update the days you already have, so you can upload in parts.
- "The file has no rows with dates" or "The file is empty." The download probably stopped early. Download it again.
Make sure the game picked on the page is the one the file is about. Wishlist uploads come with Indie and Studio.
Tip: skip the files for good. Connect Steamworks and your wishlists update by themselves every hour.