Just try to give a general idea of how grub works:
Grub has 2 stages/parts. Stage 1 is stored on MBR of your HDD (usually). Stage 2 has larger files and cannot fit onto MBR, so it's stored on a HDD partition (usually in /boot/grub).
When computer boot from your HDD, it first reads the grub stage1 info on your MBR, which will point to the location of stage2 grub image.
Grub Error 21 means stage1 is pointing to a incorrect location of stage2. And the boot will fail.
I don't know the detail steps of setting up grub, but they are available out there. Here are some links I can suggest:
Note: I wrote this purely by memory from few years ago. Things could have changed, or my memory could have failed. Grub gurus out there, please correct if I am wrong.