default or selected kernelversion does not have config value CONFIG_LNET_MAX_PAYLOAD.
Result is shown for kernelversion 6.7-rc3

Lustre lnet max transfer payload (default 1MB)

configname: CONFIG_LNET_MAX_PAYLOAD

Linux Kernel Configuration
└─>Device Drivers
└─>Staging drivers
└─>Lustre lnet max transfer payload (default 1MB)
In linux kernel since version 3.11 (release Date: 2013-09-02)  
This option defines the maximum size of payload in bytes that lnet
can put into its transport.

If unsure, use default.