Error in get(as.character(FUN), mode = "function", envir = envir) :

I am trying apply this function on a data frame to create a new feature but I keep getting this error:

Error in get(as.character(FUN), mode = "function", envir = envir) : object 'INSTALLMENT' of mode 'function' was not found

I have also tried apply(df, 2, purchase) but then I get this error:

Error: $ operator is invalid for atomic vectors

The code is :

purchase = function(DataFrame){

  if((DataFrame$ONEOFF_PURCHASES == 0) && (DataFrame$INSTALLMENTS_PURCHASES == 0))
    return('NONE')

  if((DataFrame$ONEOFF_PURCHASES > 0) && (DataFrame$INSTALLMENTS_PURCHASES > 0))
    return('BOTH_ONEOFF_INSTALLMENT')

  if((DataFrame$ONEOFF_PURCHASES > 0) && (DataFrame$INSTALLMENTS_PURCHASES == 0))
    return('ONE_OFF')

  if((DataFrame$ONEOFF_PURCHASES == 0) && (DataFrame$INSTALLMENTS_PURCHASES > 0))
    return('INSTALLMENT')
}

df$PURCHASE_TYPE = apply(df, 2, purchase(df))