I wanted to ALTER an existing table and add a new column with a foreign key constraint .
I looked at Chat GPT and also some youtube videos which showed the same syntax so I used the follwoing SQL

ALTER TABLE comments ADD COLUMN Userid INT NOT NULL;
ALTER TABLE comments ADD CONSTRAINT adding_fk FOREIGN KEY(Userid) REFERENCES Users(ID);An error message is being shown when altering a table to add a column with foreign key
#1452 – Cannot add or update a child row: a foreign key constraint fails (facebook.#sql-2c80_8ad, CONSTRAINT adding_fk FOREIGN KEY (Userid) REFERENCES users (ID)).

Why isn’t it working?