Looking at your vlookup formula the issue appears that you have not given enough possibilities to calculate your score. Formula for Letter Grade
=VLOOKUP(D10,$b$55:$C$66,1,TRUE)
B55 to C66 is only 11 possibilities but if you have scores ranging from 10% to 100% you will need a lot more. Also the lookup is looking in column 1 when I think it should be looking for the 2nd column.
I think if you fix up your range with the scores and then change your vlookup to
You would get much better results. If this dosnt quite make sense. Post a sample book and it can be fixed pretty quickly.

Thanks

EDIT:
Sorry I just reread that the score could go up to 75 points not 100 so that would need to be adjusted.