Thank you! The crucial piece of information was that I have to open via "connect to data" on the main start screen. Meaning I cannot open via legacy connection when I already have various other excel sheets loaded into Tableau?
Follow up question: As I am completely new to SQL queries I do not understand how I need to alter the code given in this link
Are the $ symbols preceded by a space?
Which of my column headers goes where?
I am trying testing things randomly like a headless chicken at this stage
For a particular data source, either the entire data source is legacy or not.
But each workbook can have a mix of legacy data sources and new data sources, if that is what is wanted.