Not sure about other ways for getting post code. May be EC can provide that. Not sure to what extent DbC can help.
For now as Angel pointed out, along with SPD, dq_map, dqs_map, Rcomp_resistor, Rcomp_Target.
I guess you have a CNL platform. Then for LPDDR4 memory, you can use
rcomp_resistor(Please verify in schematic the resistor value for DRAM_RCOMP[0..2]. I guess they should be 100 ohm resistor)
For DQS_map, you need to first find the connection between SDRAM DQS pin to CPU DQS pin map. Can you check that in schematic ?
With this setting + correct SPD, FSPM should run successfully.