No debuggable processes in logcat when phone detected by Android studio
I had a similar issue, when I was building an app, I could select the device in Logcat, but not the process. (same message, "No Debuggable Processes")
The reason: I was building my app in release, then, when I changed the build variants to debug, I was able to select the process.
You need to enable adb integration, just go to Tools-->Android-->Check Enable ADB Integration
It was a default feature a few versions ago but now it's disabled by default
In my case, using Android Studio 3.1.3, I had to enable "Use libusb backend" checkbox at File-->Settings-->Build, Execution, Deployment-->Debugger