Marco,
From the Data tab in Excel, click Show Queries in the Get & Transform section of the ribbon.
Over on the right is a list of Workbook Queries. My guess is you have a separate query for each year?
Within that list of Queries, right click on the 2015 table, then choose edit. This brings up the Query Editor in Power Query.
In Excel 2016, on the Home Tab click on the Combine dropdown. Click on the Append Queries, then choose Append Queries as New.
(In other versions of Excel/Power Query, the "Append Queries" choices are elsewhere - search for it.
The "Append Queries as New" launches a window where you can select 2 or more tables to append to one another. Append the 2015 & 2016 table: this will get you the bad result - where 2015 and 2016 data show in separate columns. IMPORTANT: Look at the formula. It should look something like this:
=Table.Combine({Table2015, Table2016})
The reason Append Queries (or actually the Table.Combine() function) fails in your situation is because your table column names are not the same. If you rename each of your column names to the generic Unit_Sales and Amt_Sales, each year's data won't be in separate columns. (Actually, for each table you append, the names and the data-types must be the same.) To retain each year's identity in the data, you must explicitly add a column to each table, such as YEAR and assign it a value such as:
= Table.AddColumn(Source, "Year", each 2015) for the 2015 table, and similarly for other years.
Rather than append tables, you can merge them and get a good result; and you don't need to rename columns.
To merge, click the Combine dropdown, but this time, select Merge Queries, then Merge Queries as New.
In Merge you must select/click the fields that match exactly - in your case, select Product column for each table.
Choose Full Outer as the Join Kind. Click OK
The formula is completely different. Here is the merge function at work:
= Table.NestedJoin(Table2015,{"Product"},Table2016,{"Product"},"NewColumn",JoinKind.FullOuter),
= Table.ExpandTableColumn(Source, "NewColumn", {"2016Unit_Sales", "2016Amt_Sales"}, {"2016Unit_Sales", "2016Amt_Sales"})
This will produce a good result for you.
Hope this helps.