With the Teensy Loader window visible and Auto mode disabled, press the button on your Teensy. Does the Teensy Loader window respond? If you unplug the cable, that window should go back to it's default "Press Button on Teensy to manually enter Program Mode".
Not sure what is different since the original post, but I am able to upload programs. However, I have to press the Teensy button every time I attempt to upload a program. To back up a bit, I first opened the Win10 Device Manager. When I connected the T3.6, I saw "USB Serial Device (COM6)" appear under "Ports (COM & LPT)". COM6 conflicts with a 3D printer listing "RAMBo (COM6)". See photo #1.
I brought up the Blink example program, then compiled & uploaded. See photo #2. I made simple changes and cycled the upload several times, and for each instance, I had to press the upload program.
Then I started a separate Linux computer (the Jetson Nano), which has the Arduino IDE installed. I opened it, brought up the Blink program, and was able to upload several times without having to press the Teensy button. I was never prompted to do so. It just worked every time.
Back to Windows 10. I then connected a brand NEW T3.6. Never connected before, until just now. The COM6 listing disappeared. A new port did appear in the Arduino IDE, under tools, under Port - the new listing was "hid#vid_16c0&pid_0486&m... (Teensy 3.6) RawHID". See photo #3. With the Blink program up, I hit the upload button and it successfully loaded the program WITHOUT having to press the button. I was not prompted to do so. I cycled this several times, and never had to press the button. After the first upload, a new port did appear in the Win10 Device Manager called "USB Serial Device (COM5)". See photo #4.
Bottom line, this upload issue is only happening on my one Teensy that happens to connect as "COM6", which conflicts with my 3D printer. Not sure how it happened. All my other Teensy devices on Windows 10 work fine (T3.2, T3.5, T3.5s). For now, I'll reserve that COM6 T3.6 for Linux - no issue there.