"Key Error: -1" while running IF statement in Python

I am trying to run the below code in a DataFrame. My intention is to include as part of the DataFrame (df) "Buy", "Sell", "Hold" or "Wait" based on the below conditions.

for i in range(len(df)):

    if df['sma_fast'][i-1]>df['sma_slow'][i-1] and df['sma_fast'][i-2]<df['sma_slow'][i-2]:
        df['Signal'] = 'Buy'  
    elif df['sma_fast'][i-1]>df['sma_slow'][i-1] and df['sma_fast'][i-2]>df['sma_slow'][i-2]:
        df['Signal'] = 'Hold'
    elif df['sma_fast'][i-1]<df['sma_slow'][i-1] and df['sma_fast'][i-2]>df['sma_slow'][i-2]:
        df['Signal'] = 'Sell'
    elif df['sma_fast'][i-1]<df['sma_slow'][i-1] and df['sma_fast'][i-2]<df['sma_slow'][i-2]:
        df['Signal'] = 'Wait'
    else:
        df['Signal'] = 'nan'

But I am getting the following error:

KeyError: -1

Could you please help me solving this issue?

Thanks!