How to add a SMA(21,Close) column to a data frame

Does anyone know how to calculate a simple moving average of 21 periods based on the close price from a stock data frame and add the SMA-21 to the data frame.

I don't know what SMA-21 is but for moving averages I would use the slider package.

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Sorry, The SMA-21 is the simple moving average for the last 21 periods based on the stock close price from the TTR package using the get_symbols method.