There may be several reasons why your printer does not recognize a specific paper or envelope size. Here are a few steps you can take to help troubleshoot the issue:

1. Confirm that your printer is compatible with the paper/envelope size.
2. Make sure the paper/envelope is correctly and securely loaded into the paper tray.
3. Minimze any other papers in the paper tray that might be preventing the printer from detecting the size.
4. Check the printer settings, confirm that the paper size setting matches what you are trying to use.
5. If necessary, try accessing additional menu options on your printer to make sure you are selecting the correct size.
6. If the options are unavailable, try reinstalling or updating the printer drivers to the latest version.