CONFIG_UML_X86 is not available for thedefaultarchitecture x86.
Result is shown for architecture um

CONFIG_UML_X86 (not configurable)

configname: CONFIG_UML_X86

User Mode Linux/$SUBARCH $KERNELVERSION Kernel Configuration
└─>UML-specific options
└─>UML_X86
In linux kernel since version 2.6.12