Exception raised when a freq cannot be null.
Particularly DatetimeIndex.shift, TimedeltaIndex.shift, PeriodIndex.shift.
Examples
>>> df = pd.DatetimeIndex(["2011-01-01 10:00", "2011-01-01"], freq=None)
>>> df.shift(2)
Traceback (most recent call last):
NullFrequencyError: Cannot shift with no freq
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https://pandas.pydata.org/pandas-docs/version/2.3.0/reference/api/pandas.errors.NullFrequencyError.html