Input volume slider says "Unavailable; use system mixer."

I, too, am new to Audacity and am having problems controlling my input level while tracking new recordings. I’m using a USB turntable which is plugged directly into my computer. I received the same message stated above which directs me to use the Mixer Board in order to adjust the tracking volume level. The mixing board, however, doesn’t increase the volume level. I’m left with a single default volume setting which is

I’ve also tried toying around with the settings for my sound card, but to no effect. Perhaps I’m doing this incorrectly? What am I missing?

I’m running the latest version of Audacity, 2.2.2, I believe, and am using the latest version of Mac OS, 10.13.4.

P.S. When I tried to plug the turntable into my PreSonus iTwo interface via the USB device port, I get no sound output at all. Nothing registers on the Audacity monitor. I’ve tried toying with the Audio Midi Setup dialogue box, switching between possible outputs, but I get no sound at all. It would probably be preferable to route the table through a preamp unless I can correct for the above issue regarding the tracking volume level within Audacity.