No sound input on iMac from Behringer UCA202


I'm an absolute beginner trying to transfer some old 12" LPs to my MacBook Pro (2013) or iMac (2009). The MBPro is useless as it has no audio input port and a TRRS jack does not show in Preferences/Sound/Input. So let's focus on the old iMac which does.

I am connecting a Technics deck with low-level output and the most success I've had is to go from that through a Pre-Amp to the UCA202 when headphone monitoring reveals a good strong signal . . BUT . . I get nothing coming into Audacity on the iMac.

I've been to Preferences/Sound/Input and selected the USB Audio CODEC input and the Output setting is to internal speakers.

In Audacity my only Input choices are Built-in Microphone or Built-in Input and I've selected the latter.

Can't understand why I have no input - can anyone help?