Unable to delete user with special character

Wrap the username in double quotes.

CREATE USER "test.user" PASSWORD 'Testing123';
-- CREATE USER

select * from pg_user where usename like 'test%';
--   usename  | usesysid | usecreatedb | usesuper | usecatupd |  passwd  | valuntil | useconfig
-- -----------+----------+-------------+----------+-----------+----------+----------+-----------
--  test.user |      121 | f           | f        | f         | ******** |          |
DROP USER "test.user";
-- DROP USER
 
select * from pg_user where usename like 'test%';
--  usename | usesysid | usecreatedb | usesuper | usecatupd | passwd | valuntil | useconfig
-- ---------+----------+-------------+----------+-----------+--------+----------+-----------