Standard SQL syntax is
DROP TABLE table_name;
IF EXISTS is not standard; different platforms might support it with different syntax, or not support it at all. In PostgreSQL, the syntax is
DROP TABLE IF EXISTS table_name;
The first one will throw an error if the table doesn’t exist, or if other database objects depend on it. Most often, the other database objects will be foreign key references, but there may be others, too. (Views, for example.) The second will not throw an error if the table doesn’t exist, but it will still throw an error if other database objects depend on it.
To drop a table, and all the other objects that depend on it, use one of these.
DROP TABLE table_name CASCADE;
DROP TABLE IF EXISTS table_name CASCADE;
Use CASCADE with great care.