Problem Recording/10.14.6 Mojave

Greetings, I don’t know if this is common, but I can’t get the audio into the computer showing on the computer’s audio display. I have the audio in set up (using a freestanding audio-digital convertor) and it recognizes the source, but when I go to set the MIDI to 44100 on the computer, the option is not available. I should say I’m trying to record from a tape deck, but that’s sort of immaterial to the issue. That’s a pretty muddled description, but I’d be happy to clarify!