How do I give the field of a!querycolumn() when querying from datastore

You have to give the CDT's field name in a!queryColumn. See the sample code for reference:

a!queryEntity(
  entity: cons!DU_DOC_TYPE_DSE,
  query: a!query(
    selection: a!querySelection(
      columns: {
        a!queryColumn(
          field: "docTypeName"
        )
      }
    ),
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 50
    )
  ),
  fetchTotalCount: false
)

You have to give the CDT's field name in a!queryColumn. See the sample code for reference:

a!queryEntity(
  entity: cons!DU_DOC_TYPE_DSE,
  query: a!query(
    selection: a!querySelection(
      columns: {
        a!queryColumn(
          field: "docTypeName"
        )
      }
    ),
    pagingInfo: a!pagingInfo(
      startIndex: 1,
      batchSize: 50
    )
  ),
  fetchTotalCount: false
)