Built - in Microphone and DGT 650 in Audacity 2.3.3

Why can’t I select the external usb microphone in the audacity program itself? Is it necessary to do so in order to use the external mic with Audacity?
WhenI go to system prefs on my mac, running High Sierra 10.13.2 and plug in the mic then the input is set up as DGT650. However Audacity says I’m using built in mic.

When I select the DGT650 in the Audacity program I get error message -9997 invalid sample rate. When I restarted the program I was briefly able to select DGT650 instead of built in mic but then I got another error message -9986 Internal Port Audio Error.

Thank you.

Hi @all,
I think I got nearly the same problem: Tried to connect an American Audio TTD-2400 USM turntable to my iMac (Mojave) with Audacity 2.4.2. On system preferences the turntable is selectable and the input level shows activity while running an LP on the turntable. Within Audacity it is selectable, too, but nothing happens by starting record. I assume there is bug within my Mac…?
Anyone with this problem, solved already?
Thanks in advance
Axel