code: def test_fillna_value_or_method(self, datetime_series):
      msg = "Cannot specify both 'value' and 'method'"
      with pytest.raises(ValueError, match=msg):
          datetime_series.fillna(value=0, method="ffill")
  
imports: from datetime import datetime, timedelta, timezone
  import numpy
  import pytest
  import pytz
  from pandas import Categorical, DataFrame, DatetimeIndex, NaT, Period, Series, Timedelta, Timestamp, date_range, isna, timedelta_range
  import pandas._testing
  from pandas.core.arrays import period_array
label_desc: Use image node_id 2 for calling the TestSeriesFillNA obj's underlying member method code with example usage: obj.test_fillna_value_or_method(datetime_series) without return types