Prepopulate using dropdown box and vlookup assistance required plz

Lulu, the problem is you have provided a very busy spreadsheet. It looks nice but you have used merged cells and they can confuse things when looking at formulas.
Now, where you have #N/A in the dashboard tab, you are trying to lookup T17 which has the word January (referenced from B22) in it and trying to look that up across the cells from U3 through BH14, then you have a 1 and false.
Vlookups look down one column and they do have a range like U3:BH14 as you wrote but when you put 1 after it that means if it finds the value in T17 in U3 through U14 it will return the value it finds in that column and ignore the rest of the range.
So, if you are wanting to know what David's total is for January you probably need a sumifs or sumproduct formula to make that work.
AND, what excel looks for is usually something that matches what is in the formula. So, the value in T17 is "January" (and it isn't a date but a text) and the list you have in cells T3 through T14 are texts too such as "jan" and "feb" etc. So if excel looks for January to match from T3:T14 it won't see "jan" as a match.

So, given all that, what do you want the results to be for David? Is it a sum of what is in U3:X3 for January? If so perhaps this sumproduct...
=SUMPRODUCT(($T$3:$T$14=B22)*(U3:BH14)*($U$1:$BH$1=T18))
NOW, that will only work if the jan in T3 is changed to match what is T17 (referenced from B22 - "January") as with the other dates, AND if you put David Vennard in each cell above the table in U1 through X1, it will return 15 for January. In other words, the values have to match what is in B21, T18 is where David's name shows up but that cell just references B21.
Hope that helps.

EDIT: I made the changes to your sheet and am reattaching it, you will see the changes in T3 and U1 through x1.