Error Referencing wp_users as Foreign Key
The MyISAM engine doesn't support foreign keys. Use the InnoDB engine instead.

Other observations:

The referenced table has no index on display_name. As per documentation, it is required to have an index on the referenced field.

It is better however to use the ID field from wp_users table to be referenced, and use this ID to lookup display_name, like:

FOREIGN KEY (user_id) REFERENCES wp_users(id)


In addition, change the field in the time_sheet table from username varchar(250) not null, to user_id BIGINT not null...