DAX command issue with IF and LEFT

[thomas_coltrain]

Amount Including Tax = If(Left(Lines1[Document Number],3) = "PCM", -1 * Lines1[Total_Amount_Incl_VAT], Lines1[Total_Amount_Incl_VAT])

Here is my command above, I am trying to look up a document number using PBI (Business Central Data) and I keep getting this error below.  Where do I put the SUM( at?  

"A single value for column 'Total_Amount_Incl_VAT' in table 'Lines1' cannot be determined. This can happen when a measure formula refers to a column that contains many values without specifying an aggregation such as min, max, count, or sum to get a single result."

Thanks for the help!

[tamerj1]

@thomas_coltrain 

This code can be used for a calculated column in the table 'Lines1' not as a measure.