Using LOOKUPVALUE in IFERROR producing an error

Hi all,

I am new to Power BI and I'm stuck in a particular scenario where I want to derive a Column Value from a MappingTable using Column_1 or Column_2 as Column references and display the corresponding values of the ResultColumn against a SearchColumn from my main table.

I am trying to populate my MainTable with a lookup value from a MappingTable, but I am trying to use two different reference columns from the MappingTable. I tried using the same approach as MS-Excel by using two lookup functions inside an IFERROR function.

Formula in Excel:

=IFERROR(VLOOKUP([@Task Name],'MappingTable'!A:G,7,0),VLOOKUP([@Task Name],'MappingTable'!B:G,6,0))

which works absolutely fine in MS-Excel.

Formula I'm trying to use in Power BI:

NewColumn(LookUp Function) =

                  IFERROR(
                        LOOKUPVALUE('MappingTable'[ResultColumn],'Task Categories'[Column_1],'MainTable'[Task Name]),
                        LOOKUPVALUE('MappingTable'[ResultColumn],'Task Categories'[Column_2],'MainTable'[Task Name])
                  )

This Expression gives the following Error Message: <A Table of Multiple values was supplied where a single value was expected.>

Note: For all the null values in MappingTable[Column_1], there exists a value in MappingTable[Column_2] which I want to use as the reference column if the Column_1 is NULL to display the Output from the ResultColumn in both the cases.

Sample Structure of my Tables:

My MappingTable looks like this:

So basically, I'm performing a lookup to fetch the value from the ResultColumn using Column_1 as reference, where if there is a NULL value, I want to use the Column_2 as reference and fetch the corresponding value from the ResultColumn.

MainTable:

Error Message:

Desired Output:

P.S: Please refrain from suggesting to merge the Column_1 and Column_2 in the first place because I just can't, I need to find out if there's anyway possible to achieve the desired result?

Any kind of input is appreciated.