PROBLEMS GETTING STARTED!

I have done quite a number of successful digitisations using Audacity in the past, but not recently. Now nothing works, just squealing acoustic feedback and when recording level is reduced no signal at all.

Details: Mac OS X 10.13.1 High Sierra
Audacity 2.1.0

I started using a brand new small Cassette player as the source, joined to iMac by the players own cable terminating in USB plug. When that didn’t work (with symptoms as above) I reverted to my old system of connecting from a high end DENON cassette player, reaching the iMac’s USB socket through a Behringer UCA22 link-up (converts two separate radio plugs to one USB cable.). This was the arrangement I have used many times when digitising LP records. In both cases the cable comes from the pre-amp sockets on the back of the amplifier in the Hi Fi setup I have.
Sadly the result was exactly the same: acoustic feedback, with no signal when recording level was reduced.

What I am I now doing wrong that I wasn’t a year ago?

At the Device Toolbar, set the input to “USB Audio Codec”.

Also, you should upgrade to the latest Audacity, which as of tomorrow should be version 2.2.1, available here
– Bill