PDF after DeDRM works, but not AZW3

Hello

I just DeDRM'd a book, it's a PDF. When I loaded it onto my device it works perfectly fine, just the letters are a bit faint because of how the PDF is.

I wanted to convert it to AZW3. I did, and it opens on Calibre. But when I load it to my device via USB, I open the file then I get a warning that says:

"An error occured. If you purchased this item from Amazon, remove the downloaded item and download it from the Cloud."

I don't know why it does this, it's very frustrating.

Does anyone know a fix?