|You can't use an IF to find values over a range of cells.|
Assuming you are trying to match values in Column A and return the value from Column B, try something like this.
Note: Since I can't see your worksheet from where I'm sitting, I might not have the formula exactly correct. Review the Excel Help files for VLOOKUP to see if it sounds like what you need.
VLOOKUP(lookup_value, table_array, col_index_num, range_lookup)
=VLOOKUP(A2, 'PT Vendor List'!$A$2:$B$748, 2, 0)
This will look through 'PT Vendor List'!$A$2:$A$748 for the value in A2 and return the value from the second column of 'PT Vendor List'!$A$2:$B$748, which is column B.
If the value isn't found it will return #N/A, which can also be dealt with. However, before we get into that, determine if VLOOKUP will fit your needs.
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