[Python] ValueError: Inferred frequency None from passed values does not conform to passed...

I am trying to create month frequency index for time series analysis The date variable is a timestamp starts from 2018-10-18 10:50:10 till 2019-11-25 15:09:33. I am getting the error in subject line i tried removing the nulls but that did not help.

df = pd.read_excel("Source_Data.xlsx", index_col="activation date", parse_dates=True)

df.rename(
columns={
"activation date": "Date",
"Estimate Volumes in HL (ABI)": "Volume"
},
inplace=True
)

dataset = df.filter(["Date", "Volume"], axis=1)

dataset.index.freq = "MS"

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