Can't not see the popup "Library" oon Audacity.

Good evening. I have the last versione from UBUNTU 16.04 LTE with AMD64. I have charged the last version on Audacity from source code.
My question is: " Why i can’t see the popup “Library” in preference? When i save the new file mp3, Audacity automaticly save in format .aff, why?
I wait an answer from you, please. Thanks a lot for all, by.

The “Libraries” option is only visible when dynamic linking is enabled.
On Linux, Audacity is usually built with dynamic linking disabled, so that Audacity can link to the system libraries for LAME and FFmpeg (determined by the compiler options --disable-dynamic-linking and the choice of system or local libraries)

The automatic file extension selection is currently (partially) broken on Linux. It broke with the upgrade from Wx2.8 to Wx3 and it’s still waiting to be fixed (patches welcome). The workaround is to manually type the required file extension.

If you want to check that exported files are in the expected format, I’d recommend using “MediaInfo” (available in the Ubuntu repository).