How do i create an envelope?
There is more to the list than your screen is allowing. This is indicated by the triangle pointing down at the bottom of the list. Either use the arrow down key to scroll through the list or use the mouse pointer on the triangle to expose more of the list:
Documentation for envelopes - see Writer Guide Chapter 5 - click here.
If this answers your question please click on the 
(upper left area of answer).
I neglected to mention that when I clicked the down pointing triangle I got an error message telling me that some part of OpenOffice was missing or moved or something. OpenOffice has not been installed on my PC since I started using LibreOffice.
Now, however, I have learned how to navigate with the down arrow key to get to the envelope. THANK YOU VERY MUCH. I tried to search the documentation but could not find anything useful. Now I am going to follow your link to Writer Guide Chapter 5.