error when inputting to DB

Based on the error it seems that you are not providing a value to some column that is defined as non-nullable in database table and it may most likely be the primary key of the table. The expression should be as follows -

 a!writeToDataStoreEntity(
  dataStoreEntity: cons!AS_DataStore_Cart_Pointer,
  valueToStore: /* local variable or rule input of CDT type which corresponds to data store entity above */ 
)

See below an example of how to use the Write to Data Store Entity Smart Service Function on an Interface -

https://docs.appian.com/suite/help/21.3/recipe-use-the-write-to-data-store-entity-smart-service-function-on-an-interface.html

I just changed to a!writeToDataStoreEntity(cons!AS_DataStore_Cart_Pointer,{1,1,1,1,1}) to test it out and its gives the same error, where should be the problem then?

Instead of giving values directly, set them by constructing the CDT using those values or use local variables or rule inputs of that CDT with those values. I am not sure what is the structure of your CDT, but for e.g. if there is an employee cdt with 3 fields (id, name, phone) you can use -

a!writeToDataStoreEntity(
  dataStoreEntity: cons!EMPLOYEE_ENTITY,
  /* just start typing type!your_cdt_name and appian will provide list of cdt's to choose from for the type constructor */
  valueToStore: type!<namepace><cdt_name>(id: 1, name: "abc", phone: "1234567890")
)

used local variables and it worked, still cant fully understand why but it does thank you

used local variables and it worked, still cant fully understand why but it does thank you