Yahoo data downloader does not download any data??

- PG: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- TRV: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- UNH: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- CRM: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- VZ: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- V: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- WBA: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- WMT: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- DIS: No data found for this date range, symbol may be delisted
[*********************100%***********************] 1 of 1 completed

1 Failed download:
- DOW: No data found for this date range, symbol may be delisted
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
[](https://localhost:8080/#) in ()
15
16
---> 17 df = YahooDownloader(start_date = TRAIN_START_DATE,
18 end_date = TEST_END_DATE,
19 ticker_list = DOW_30_TICKER).fetch_data()

[/usr/local/lib/python3.9/site-packages/finrl/meta/preprocessor/yahoodownloader.py](https://localhost:8080/#) in fetch_data(self, proxy)
58 num_failures += 1
59 if num_failures == len(self.ticker_list):
---> 60 raise ValueError("no data is fetched.")
61 # reset the index, we want to use numbers as index instead of dates
62 data_df = data_df.reset_index()

ValueError: no data is fetched.