MySQL is not supporting single line comments here. What would be the reason?

From MySQL documentation:

From a “-- ” sequence to the end of the line. In MySQL, the “-- ” (double-dash) comment style requires the second dash to be followed by at least one whitespace or control character (such as a space, tab, newline, and so on).

You need some space character after --, for example:

mysql> select 1;-- test select

Your syntax is wrong - read about comment syntax.

Just add a space after --:

mysql> select 1; -- test select
+---+
| 1 |
+---+
| 1 |
+---+
1 row in set (0.00 sec)