To use VLOOKUP function you have to enter at least the following arguments:
=vlookup(lookup value, table_array, column_index_number, range_lookup)
You should check if the table array is set correctly. For example when you need to get the values from the third column in table array, check if you have enough columns in the table array range.
If the range of table array is for example C2:D10, which means there are 2 columns, you can't set column index number to 3, because there are not enough columns. When the column index number is greater than table array range, you get an invalid cell reference error with #REF! in the cell.