Error code 1215: cannot add foreign key constraint - mysql

Question

I am trying to add a foreign key constraint, but it fails. I did that many times before and I can not determine why it is doing that. Basically, I want to relate Skills and Employees to the Skill_Bridge table. I was able to relate Skills with the Skill_Bridge, however when I try to do the same with Employees it fails. The data types are the same, so I do not think this is the problem. I also tried to creating a primary key for the Skill_Bridge, and then try to relate them and it did not work as well. The first constraint is the one that fails 'FKey1'. This is my code. Any help will be appreciated. Thanks in advance.
create database if not exists Q3;
use Q3;
drop table if exists Employees;
create table Employees(
employeekey int not null,
firstName varchar (100) not null,
lastName varchar (100) not null,
employeeSkillGroupkey int not null,
primary key (employeekey) );
insert into Employees values
(1, 'Ted', 'Codd', 1),
(2, 'Ralph' ,'Kimball', 7),
(3, 'Joe' ,'Celko', 1),
(4, 'James' ,'Gosling', 2),
(5, 'Godfrey', 'Muganda', 6),
(6, 'Margy', 'Ross', 5),
(7, 'Peter', 'Chen', 4),
(8, 'Terry' ,'Halpin',3),
(9, 'Tony', 'Morgan', 2);
drop table if exists Skills;
create table Skills(
empSkillKey int not null,
empSkillDescription varchar (1000) not null,
empSkillCategory varchar (200) not null,
primary key (empSkillkey));
insert into Skills values
(1, 'SQL', 'Database'),
(2, 'ERD', 'Database'),
(3, 'DM', 'Database'),
(4, 'Java', 'Programming'),
(5, 'Pascal', 'Programming');
drop table if exists Skill_Bridge;
create table Skill_Bridge(
employeeSkillGroupkey int not null,
empSkillKey int not null
);
insert into Skill_Bridge values
( 1, 1),
( 2, 4),
( 3, 4),
( 3, 5),
( 4, 4),
( 4, 2),
( 5, 1),
( 5, 3),
( 6, 4),
( 6, 5),
( 6, 2),
( 7, 1),
( 7, 2),
( 7, 3),
( 7, 4);
ALTER TABLE Employees ADD CONSTRAINT FKey1 FOREIGN KEY (employeeSkillGroupkey)
REFERENCES Skill_Bridge (employeeSkillGroupkey)
ON DELETE Restrict
ON UPDATE Cascade;
ALTER TABLE Skill_Bridge ADD CONSTRAINT ForK2 FOREIGN KEY (empSkillKey)
REFERENCES Skills (empSkillKey)
ON DELETE Restrict
ON UPDATE Cascade;

Answer 1

You need to create an index on employeeSkillGroupkey in skill_bridge table before you can reference it in a foreign key, see mysql documentation on foregn keys:
InnoDB permits a foreign key to reference any index column or group of
columns. However, in the referenced table, there must be an index
where the referenced columns are listed as the first columns in the
same order.
You may also want to reverse the direction of the foreign key. The connection table should reference the master table, not vice versa.