Credit this to philschroeder1 on another question. This worked for me after costing me an entire day of frustration.
SysPrefs>Accessibility>Privacy>Microphone: Make sure Studio One app is in there and box is checked.
StudioOne was there & the box was not checked. Allowed the microphone & BOOM! Now getting signal in S1Pro input on the correct channel. Previous to turning this on: I was getting good signal on the Quantum & in Universal Control from my condenser mic with 48v phantom power on the channel, just no input signal in S1Pro software, either in the input section or on the channel with the "listen" & "record ready" icons highlighted. Many thanks to philschroeder1 for saving the day. Presonus really needs to spell this out for people. Never had this problem with my Presonus mixers or UA Apollo Twin using S1Pro.
Longtime Presonus S1Pro user with FaderPort, StudioLive 1602 & 16AI mixers, HP6, AudioBox2 with various MacMini's, MacBookPro's, & iMacs