Some of the "numbers" in column A are proper numbers, but some of them (especially near the bottom of the list) are actually text values which just look like numbers. You can spot these if you widen column A - proper numbers will be aligned to the right by default, whereas text values are aligned to the left.

You can convert the values to proper numbers using Text-to-columns. Alternatively, you can change the formula in D2 to this:

=IFERROR(VLOOKUP(D1,A1:B3810,2,FALSE),IFERROR(VLOOKUP(D1&"",A1:B3810,2,FALSE),""))

Hope this helps.

Pete