Hi,
It appears that the J-Link can connect to qspi flash directly, but the the LPC54018JxM has internal flash, so the J-Link device can not connect to quad spi flash directly, so you can not use JFlash to program internal flash of LPC54018JxM directly.
You can use blhost to program internal flash of LPC54018, pls refer to the doc
Hope it can help you
BR
XiangJun Rong