default or selected kernelversion does not have config value CONFIG_AD9951.
Result is shown for kernelversion 5.9.9

Analog Devices ad9951 driver

modulename: ad9951.ko
configname: CONFIG_AD9951

Linux Kernel Configuration
└─> Device Drivers
└─> Staging drivers
└─> IIO staging drivers
└─> Direct Digital Synthesis
└─> SPI support
└─> Analog Devices ad9951 driver

Say yes here to build support for Analog Devices DDS chip
ad9951, provides direct access via sysfs.

source code: