Hi @Andrzej
Welcome to Microsoft Q&A platform and thanks for posting your query here.
I have reproduced the scenario from my side with simple CSV file
I am able to add column without error
I am getting the columns in dynamic expression as well
Kindly check the below steps and confirm us if issue resolved.
- Double-check the column mappings in your source settings. Ensure that the column names in your CSV file match exactly with what you are referencing in the Derived Column transformation.
- If the columns are not visible in the Dataflow Expression Builder, try refreshing the schema or re-importing the source dataset. Sometimes, a refresh can help in recognizing the columns.
- Ensure that the data types of the columns are correctly defined. Mismatched data types can sometimes cause issues with column recognition.
I hope the above steps will resolve the issue, please do let us know if issue persists. Thank you