Error while combining and transforming excel files

Hi everyone,

I was trying to combine and transform multiple excel files which had names as: SalesTarget_2015, SalesTarget_2016, SalesTarget_2017, SalesTarget_2018. I wanted the result to look like this:

However, I was left with this:

(It said there had been an error in the "Transform File" query that made the rows from SalesTarget_2016, 2017 and 2018 woulddn't show up)

And the "Transform File" code was like this:

let
Source = (Parameter1) => let
Source = Excel.Workbook(Parameter1, null, true),
SalesTarget_2015.csv_Sheet = Source{[Item="SalesTarget_2015.csv",Kind="Sheet"]}[Data],
#"Promoted Headers" = Table.PromoteHeaders(SalesTarget_2015.csv_Sheet, [PromoteAllScalars=true])
in
#"Promoted Headers"
in
Source

Is there anyway I can change it to get my desired result?