Thank you @szymon_dybczak for the response.

As you can see I have already defined the not null constraint in my definition for the primary key x_key
CONSTRAINT pk_key_not_null EXPECT (x_key IS NOT NULL) 

But still I am getting the same error.Also I checked in source to check if we have any null values but we dont...and UAT and DEV have same data set.. 😞