When you want to store a date, use a DATE or DATETIME data type. Using VARCHAR for dates is not a recommended practice.
If you do use VARCHAR, always specify a length. E.g. VARCHAR(32).
The following code should produce what you are asking for.
DROP TABLE IF EXISTS #CSV_Convert;
CREATE TABLE #CSV_Convert
(
[Store_no] [bigint] NULL,
[Date] Date NULL
);
INSERT INTO #CSV_Convert
VALUES('16','20200307')
INSERT INTO #CSV_Convert
VALUES('17','20200305')
SELECT
Store_no,
Date
FROM
#CSV_Convert FOR XML PATH('Store'), ROOT('Stores');
This relies on default conversion of Date to string. If that is not the format you want for date, you can use CONVERT function, i.e, replace the "Date" just before the "FROM" clause with CONVERT(VARCHAR(32),Date,23) AS Date