如何将带有YearMonth列的数据框转换为R中的时间序列

I have the below dataset which is a dataframe. But I would like to convert it into time series so that I can do ARIMA forecasting. Have searched various topics in SO but could not find anything similar which is at YEARMONTH grain. Everyone talked about date field. But here I don't have date. I am using the below code but this gives error I have the below dataset which is a dataframe.