@omemanti @mfalkvidd Thank you, I think I did it. It works now - or I hope so from the code.
** Programming Finished **
** Verify Started **
nrf51.cpu: target state: halted
target halted due to breakpoint, current mode: Thread
xPSR: 0x61000000 pc: 0x2000002e msp: 0x20004000
verified 1412 bytes in 0.038633s (35.692 KiB/s)
** Verified OK **
** Resetting Target **
shutdown command invoked
What I was confused is that there is erase sketch, but to make it erased there is this trick with burn bootloader. Still the port is greyed out, but it seems I am able to flash blank sketch. Thanks