It looks like you are using a Mega-5X version that does not support all of your axes.
When installing GrblGru, in the directory Program(x86)\toe\GrblGru xxx
the file 'grbl-Mega-5X-v1.2d_XYZAB_SPINDLE_PWM_ON_D6.hex' is
stored.
Use the program 'XLoader' to flash your Mega-5X controller with this file.
This provides the axes X, Y, Z, A and B.