You have a few problems here,

Why bother with the Trim function on C6 ?
You need to lock the ranges in the formula with the $ so they don't change as you drag the formula down.

Vlookup can't do the task, because it appears your matching values are in column F, and you want to return a value from column A
Vlookup can't do that, it can only find match in A and return value from F.

Try
=IFERROR(INDEX('Account Detail (2)'!$A$3:$A$6,MATCH(C6,'Account Detail (2)'!$F$3:$F$6,0)),"")