The best way to flash Marlin via PlatformIO onto the LPC1768 MCU on a BTT SKR v1.4 board is to insert an SD card into it, connect the computer via USB and use the mbed as upload_protocol.
In order to do this, edit ini/lpc176x.ini from within Visual Studio Code and add the following new code to [env:LPC1768]:
upload_protocol = mbed upload_port=/media/uli/A87B-A154/
with upload_port being the directory where the SD card is mounted (while you can do this using an SD card reader, it is so much easier by just connecting the BTT SKR v1.4 via USB directly to your computer, allowing both serial port and SD card access at the same time).
Full example of the [env:LPC1768] section:
[env:LPC1768]
platform = ${common_LPC.platform}
extends = common_LPC
board = nxp_lpc1768
upload_protocol = mbed
upload_port=/media/uli/A87B-A154/In the PlatformIO menu, choose LPC1768/Upload. There is no special configuration for the BTT SKR v1.4 but of course you need to configure Configuration.h etc correctly – see https://www.makenprint.uk/3d-printing/3d-printing-guides/skr-v1-4-configuration-h-marlin-2-setup-part-2/
After uploading, press the reset button on the Board to apply the firmware update. You can use picocom to connect to your printer, see How to connect to your 3D printer using picocom, e.g.:
picocom -b 115200 /dev/ttyACM0 --imap lfcrlf --ech
Enter M115 to check if you have correctly updated the fiwa
You might need to manually re-mount the SD card using your file manager after a firmware update in order to enabled PlaformIO doing another update.