Dropdown value to database

I got a form in a table that is submitted on blur of a select input/dropdown menu from which a category can be selected from a database list:

image458×1183 27.8 KB

I get an error from the SC action that there is no value to be updated:
message: "Empty .update() call detected! Update data does not contain any values to update. This will result in a faulty query.

I can’t figure out how to set the selected dropdown option as a value for the form and send it to the SC action on submission. How can I achieve that?