Hi @NGatensby ,
Do you want to add a column in Excel and update all the data in this column to a certain data?
You need two flows to do this, please try this:
The first flow for adding a new column:

The second flow for updating rows:

Hope this helps you!
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If my answer solves your problem, please accept it as a solution to help more people stuck on the same problem find it.
Best regards,
Rimmon Li