I had the same issue when creating a Nexus 6 AVD with an x86_64 image and it appears the HAXM does not support that on an old Core 2 Duo (Mac Book Pro late 2009 for example).
This is specified in the Release note known issues in $ANDROID_SDK_HOME/extras/intel/Hardware_Accelerated_Execution_Manager.
HAXM driver does not support emulating a 64 bit system image on Intel
systems based on Core microarchitecture (Core, Core2 Duo etc.). All
systems based on Nehalem and beyond are supported. (Corei3, Core i5
and Core i7 machines).