See this question: Converting data.frame to xts order.by requires an appropriate time-based object, which suggests looking at argument to order.by,
Currently acceptable classes include: ‘Date’, ‘POSIXct’, ‘timeDate’, as well as ‘yearmon’ and ‘yearqtr’ where the index values remain unique.
And further suggests an explicit conversion using order.by = as.POSIXct,
df$Date <- as.POSIXct(strptime(df$Date,format),tz="UTC")
xts(df[, -1], order.by=as.POSIXct(df$Date))
Where your format is assigned elswhere,
format <- "%m/%d/%Y" #see strptime for details