What is the type of "Notes" column?
Based on your description, I understand you create a "Notes" column in the list, then in the task action of a workflow, you cannot find the "Notes column" in the "Field of source".
Per my test, when I create a multiple lines of text column, then I can find "Notes" column in the "Field of source".
Please restart the SharePoint Designer, then have a test, compare the result.
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