If you are not seeing any signal on the channel that you are trying to record, you might not have the inputs correctly set. Be sure they match with what you have plugged into your audio interface. Also, check your Audio Settings and make sure your Scarlett is selected for both input and output. 
Finally, there's an odd setting that can mess up audio inputs for Logic Pro, but it isn't actually in Logic. This is one thing that has caused many Logic users grief. And easy to remedy.
Check the Control Center on the top right of MacOS menu bar. It's where your volume control, network setting etc are located. You should see a microphone while Logic is running. Click on it and change to standard.
Hope this helps.
