VLOOKUP matches the value in the leftmost column of the stated range, and return the matching row value in the column 'n' to the right, based on the third argument of the formula...
Your VLOOKUP formula is trying to match a number with a Month string, in [Date List] column A - of course, it can't find it, so it returns #N/A.
If you are just using that number (in B10) to return the month name from column A of [Date List], then why not use an INDEX:
In cell C10 enter
Formula:
=INDEX('Date List'!A2:A72,B10)
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