I tried to send a document by e-mail from Libre Office and I could not because I do not have an e-mail program associated with Libre Office.
Have you tried creating an e-mail with your existing e-mail program and then attaching your LibreOffice document to it, as an “Attachment”? That is what most people do.
Note that LibreOffice does not have to be “associated” with any other program. You only have to use your operating system’s file manager to find your documents, most of which are stored in your My Documents folder.
do not have an e-mail program associated with Libre Office.
That might be completely pointless, if you are using Windows (but - quite as usual here - no tech details at all).
You need to have an existing email program that is set as default program so it will open when you click on a mailto: link.
If you are using web mail you can set up a program like Thunderbird to collect and send emails instead. If you use setting of IMAP the email program will leave copy of mail on email server so other devices can also stay in synch with all mail sent and received.