Hi,
As you know the LPC54018 does not have on-chip flash, you have to save code in spifi flash, then copy to SDRAM and execute from SDRAM.
I attach an application note an12423.pdf which discusses where/how to initialize SDRAM and MPU so that code can be copied into SDRAM and execute from it.
.https://www.nxp.com.cn/docs/en/application-note/AN12423.pdf
Hope it can help you
BR
XiangJun Rong