Error creating foreign key from MySQL Workbench
You will get this message if you're trying to add a constraint with a name that's already used somewhere else.

According to this post you can check your constraint like this:

If the table you're trying to create includes a foreign key constraint, and you've provided your own name for that constraint, remember that it must be unique within the database. Run this query to see if that name is in use somewhere:

SELECT
  constraint_name,
  table_name
FROM
  information_schema.table_constraints
WHERE
  constraint_type = 'FOREIGN KEY'
  AND table_schema = DATABASE()
ORDER BY
  constraint_name;