Thanks for your question. The reason for this notification is that there is a gap in the time series data.
From the competition description, I can see that the training data is only for the first 19days of the month and the test period is from the 20th to the end of the month. You can go ahead and use row-based time series here as the problem statement is predicting part of the month.
But in some cases when the data is not available and if you want to generate those data points then you can use the inbuilt data prep tool for time-series which will help you fix those missing data points - Refer to this doc link for the same - TS Data Prep. Let us know if this answers your question and if yes please mark this as the accepted solution.