Alternatively, you can also delete a Foreign Key Constraint from the SQL Server Management Studio itself. You can try it if the commands do not work.
- Expand your database view.
- Right Click on Table which has foreign key constraint. Choose Design. A tab with the information about table columns will open.
- Right click on the column which has the foreign key reference. Or you can right click on any column. Choose Relationships.
- A list of relationships will appear (if you have one) in a pop up window.
- From there you can delete the foreign key constraint.
I hope that helps