Hi,
I suppose that you use NXP manufactured LPCXpresso804 boards, if it is the case, you have to disable on board debugger by closing the JP1 and connect the SWD signal to CN1 of LPCXpresso804 boards.

SWD connector CN1 signals assignment

On the IDE, you can delete the *debug.lauch file so that the tools can research the device.
If you use your own board, I am sorry.
Hope it can help you
BR
Xiangjun Rong