If you change the Function to this
=VLOOKUP(AC6,Leases!K:L,2,FALSE)
this should work for you
I just replaced 12 with 2
Although L is the 12 letter of the alphabet you are only looking up 2
Columns K and L so the VLOOKUP you suggested was trying to find the
12th Column in the range K:L which dosen't Exist hence #REF! is returned.
Hope this helps
David