You need to specify the columns of the "return table", this is either done using
returns table(col_1 integer, col_2 text, ...)
In your case you are returning only rows of one table, so it's easier to use
returns setof maintable
As documented in the manual the function body needs to be enclosed in single quotes, or using dollar quoting.
As stored functions can be written in many different languages in Postgres, you also need to specify a language - in this case language sql
is suitable.
So putting all that together, you need:
CREATE FUNCTION schemaB.testFunc(p_id INT)
RETURNS setof schemaZ.mainTable
AS
$$
SELECT *
FROM schemaZ.mainTable
WHERE id = p_id
$$
language sql;
A return
statement is not required for language sql
functions.