how can i fix error: "Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex, but got an instance of 'Index'"?

Question

I have a dataset containing various columns of created_at, text, author.id, i am trying to do a time analysis:

  df['date'] = pd.to_datetime(df['author.created_at'])
    df = df.set_index('date') 
    timestamp = (df_time!=0).resample('AS').sum()

I am getting error as:

TypeError: Only valid with DatetimeIndex, TimedeltaIndex or PeriodIndex, but got an instance of 'Index'

Can anyone help me with this?Thank you