Anyone else run into this issue or am I doing something wrong?

I have a table which an end user wants pivoted by date, such that the column headers will be every month in 2018, the rows are expense types, and the values are sums of $.

I am able to get the dataflow preview to run successfully, but am unable to add more columns. In order to select a new column to create, I need to select a date, which is fine, as all the dates in my source are the last days of a given month, no variability. However, the "Add Column" button is greyed out.

Seeing as the preview ran successfully, it's odd that I can't add more columns. Had I done something wrong I would imagine the preview would not run.