SQL - error code 1005 with error number 121

Error 121 means that there is a foreign key constraint error. Since you're using InnoDB, you can use SHOW ENGINE INNODB STATUS after running the failed query to get an explanation in the LATEST FOREIGN KEY ERROR section. Having run your SQL myself, I get this:

------------------------
LATEST FOREIGN KEY ERROR
------------------------
101210 14:55:50 Error in foreign key constraint creation for table `regula`.`Reservation`.
A foreign key constraint of name `regula`.`prjId`
already exists. (Note that internally InnoDB adds 'databasename'
in front of the user-defined constraint name.)
Note that InnoDB's FOREIGN KEY system tables store
constraint names as case-insensitive, with the
MySQL standard latin1_swedish_ci collation. If you
create tables or databases whose names differ only in
the character case, then collisions in constraint
names can occur. Workaround: name your constraints
explicitly with unique names.

Basically, you need to give your prjId constraint name a unique name in the last table. Constraint/foreign key names are global to a database, so they cannot be reused in different tables. Just change the last

  CONSTRAINT `prjId`

to

  CONSTRAINT `prjId2`

The error code 121 comes when you try to map the foreign key.

Basically it comes when your foreign key name already exists in the database.

For example:

ALTER TABLE `photokiosk`.`kiosk_event`
    ADD CONSTRAINT `event_booking_id`
    FOREIGN KEY `event_booking_id` (`event_booking_id`)
    REFERENCES `event_booking` (`event_booking_id`)

If foreign key with the name event_booking_id is already mapped with the other table.

To get rid of this issue, please change the foreign key name, not the column name.