The internal name of a column is a space of eight characters and non-alphanumeric characters.
or to find the internal column name, just click the column name in the list and check the url value after SortField = is the internal column name.
eg. in my case, usrl looks like this: http: // [server] / [sitecollection] / [site] /listname/Forms/AllItems.aspx?View= {see GUID} & SortField = TradingS & SortDir = Asc
and my internal field name is "TradingS".
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