You may have entered an invalid symbol. OTC stocks (many with five-letter symbols), mutual funds, indexes and stocks on foreign exchanges are not supported.
A stock symbol that includes a hyphen (-) is not supported. Use an underscore (_) instead. For example, the symbol for Berkshire Hathaway is BRK_B.
If the input form has date selections, the dates could be bad. For example, the start date could be after the end date.
Finally, if you are sure all input data are valid, try again. Occasionally price and/or dividend data are not downloaded correctly.