I had this error just recently. I did basically the same table transformation 4 times over the course of 2 days. Added two new fields to two different tables using the same SQL syntax. It worked 3 out of 4 times, but for 1 the column is not available...
I found I also have to do dbutils.widgets.remove('Var1') in one cell and dbutils.widgets.text('Var1', originalValue + ", World!") in the next cell or it remembers the old value when it sets the widget back up.
