default or selected kernelversion does not have config value CONFIG_SND_MMAP_SUPPORT.
Result is shown for kernelversion 5.3.7

Enable MMAP Support

configname: CONFIG_SND_MMAP_SUPPORT

Linux Kernel Configuration
└─>Device Drivers
└─>Sound card support
└─>Advanced Linux Sound Architecture
└─>ALSA for SoC audio support
└─>Enable MMAP Support
In linux kernel since version 2.6.28 (release Date: 2008-12-24)  
Say y if you want AC97 driver to support mmap mode.
We introduce an intermediate buffer to simulate mmap.