I have installed "Armbian_5.54.180727_Nanopineo_Debian_stretch_next_4.17.11" in the eMMC..My doubt is always boot from eMMC, If blank sd card is inserted in the slot.

Of course, as long as there is no u-boot sector present in the SDCard, it is not sufficient to format the current partition since u-boot is located outside, usually at location 8192.

 

how to set primary boot from eMMC in "Armbian_5.54.180727_Nanopineo_Debian_stretch_next_4.17.11" installed nanopi neo core..not boot from sd card

You can't change boot order, SoC look first at SDCard, then eMMC, and last at SPI-NOR.

To make it boot from eMMC, the SDCard needs to be non-bootable, so that why I've said that "no u-boot sector should be present on SDCard".

You can erase it with :

dd if=/dev/zero of=/dev/mmcblk0 bs=1024 seek=8 count=512

 

Thank you