What extension i need to save a document

My computers saves it with Microsoft word and Excell. I don’t know what extension to use. Then I can’t open it with Libre
because it was created with Excell. Other words the windows software interfaring with Libre

To open your existing documents, open LibreOffice, in the menu click File > Open... (or Ctrl+O), in the window that opens navigate to your Excel or your Word file, select it and click Open button.

Alternatively, in File Explorer right-click on the Word file you want to open and choose Open With > LibreOffice Writer. If you don’t see LibreOffice Writer then click on Choose another App. In the new window that opens, select LibreOffice Writer, you might need to scroll to find it. If you no longer have Word you can tick the box that says “Always use this app to open [filetype] files”

LibreOffice will default to saving Writer files as OpenDocument Text with the extension .odt (which you might not be able to see in File Explorer if you have Hide Extensions for known file types ticked). LO Calc will default to OpenDocument Spreadsheet, .ods .

Or is it that you can’t save the file at all? If so then see question 260833