MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE messages (messageid INTEGER PRIMARY KEY AUTO_INCREMENT
                       , message VARCHAR(150)
                       , sender VARCHAR(40)
                       , recipient VARCHAR(40))


CREATE TABLE employees (employeeid INTEGER PRIMARY KEY
                        , employeename VARCHAR(40)
                        , area VARCHAR(40))
insert into employees values (1, 'Mark', 'Maintenance')
insert into employees values (2, 'Ann', 'Maintenance')
INSERT INTO messages (message
                      , sender
                      , recipient) 
VALUES ("Message."
        , "System"
        , (SELECT employeename 
           FROM employees 
           WHERE area = "Maintenance"))
Subquery returns more than 1 row

select * from messages