Issue : waiting for bootloader... Case : waf upload of arducopter build to px4 2.4.6

I was trying to upload arducopter build (px4-v2 also tried v3, v4) to a PX4 2.4.6, usiing waf on a Ubuntu 18.04. The px4 is connected through a USB, connected to port /dev/ttyACM0. Every time I try to upload it gets struck at the following point.

$ ./waf --targets bin/arducopter --upload
Loaded firmware for 9,0, size: 1085176 bytes, waiting for the bootloader…
If the board does not respond within 1-2 seconds, unplug and re-plug the USB connector.

Reconnecting doesn’t seem to make it respond. Please could anyone point me in the right direction ? Need more details ?
Thanks in advance

Have you found the answer to this one?