Expression.Error: The column 'X' of the table wasn't found.

A step references a column that doesn't exist at that point — usually renamed, removed, or a source schema change upstream.

Why it happens

• An upstream rename or removal broke a later step that still names the old column.
• The source added, removed or reordered columns since the query was built.
• A 'Promoted Headers' or 'Changed Type' step changed the column names.

How to fix it

1. Open the failing step in the Advanced Editor and re-point it to the current name.
2. Make selection resilient: Table.SelectColumns ( Source, {"A","B"}, MissingField.UseNull ).
3. Where a source's schema drifts, add a rename/normalisation step early so downstream steps are stable.

Table.SelectColumns ( Source, {"Page","Sessions"}, MissingField.UseNull )