SELECT
(SELECT COUNT(*) FROM table1) AS count1 WHERE date='2019-06-12',
(SELECT COUNT(*) FROM table2) AS count2 WHERE date='2019-06-12'
why is this statement causing a syntax error at or near ","?
Let's rewrite the statement replacing the SELECT in parenthesis with a simple value:
SELECT
1 AS count1 WHERE date='2019-06-12',
2 AS count2 WHERE date='2019-06-12'
Now it's easy to see that up to the comma you have a valid SQL query, but you're appending further values that you want to select, which is invalid.
I assume what you want is to have the WHEREs inside the subqueries:
SELECT
(SELECT COUNT(*) FROM table1 WHERE date='2019-06-12') AS count1,
(SELECT COUNT(*) FROM table2 WHERE date='2019-06-12') AS count2