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TR.TRBCECONOMICSECTOR should be used with get_data() function.
You can use "Data Item Browser" to check supported parameters.
TR.TRBCECONOMICSECTOR does not support timeseries so you can only get the current value.
ek.get_timeseries(rics,
start_date='2017-01-01',
end_date='2018-02-23',
fields='TR.TRBCECONOMICSECTOR',
interval='MONTHLY'
)
does not seem to work