
Your formula is looking for the wrong value. W2 does not contain the numeric grade to translate. Try
=VLOOKUP(V2,Y$6:Z$10,TRUE)
The $ qualifiers are necessary so when you copy the formula to lower cells the lookup range is unchanged.
Note that this technique will give the same grade to 86% and 99%. 31% becomes a D instead of an F. Is that your intent? You may want to consider XLOOKUP with a match_mode of -1.