Practically identical topic as in
This external 3rd-party developed LPC platform at GitHub - p3p/pio-nxplpc-arduino-lpc176x that Marlin is using can’t handle upload_protocol = stlink, not PIO itself, to be exact.
If you get it to work based on the topics above please let us know, noone has followed through with this yet. You’re already pretty close since you can connect to the target – if you have a compiled .bin firmware you might want to try the -c "program .." commands listed above.
You might however find it significantly easier to just upload the firmware via the bootloader if it still has one.