looking at you code, you sdont have the insert and manage relational table configured properly.
both the insert and MRT are using the same premium_hrs table.
The use of the MRT behavior is to manage a many to many remationship using a linking table.
as an example, lets take products and options.
the table would llok like:
productID - Primary Key
optionID - primary key
productoptionID - primary key
productoptionProductID - relates to products.productID
productoptionOptionID - relates to options.optionID
In this scenario, the insert would insert a product to the product table.
In the form, your checkboxes would show the available options.
The MRT behaivor would manage the productoptions table, it would insert the productID value to the productoptionProductID column and the option ID from the checkboxes to the productoptionOptionID column