I was unable to select my Headset Microphone Input in the Squad Audio Settings. I was able to resolve this by going into my Ubuntu Settings > Apps > Steam > Record Audio and turning on the Record Audio permission for Steam. I also installed the PulseAudio Volume Controller by running sudo apt install pavucontrol in terminal, and disabled all audio devices other than my headset. I was then able to select my Headset Output and Microphone Input in the Squad Audio Settings which resolved the problems.