In the test data part of your time series, you have six timestamps and the values are respectively
10, 8, 7, 8, 10, 12
You used Holt-Winters method using the training part. The forecasted values using Holt-Winters method for six timestamps are:
9, 9, 8, 7, 11, 11
What is the RMSE of your forecast? Your answer must be correct up to three digits after the decimal point.