Pupils leaving date is either missing, is either before entry date or equal to or after census date.
Go to the Enrolment section of the student's profile and check the start and end dates for their enrolment into your school.
For example, a student is flagging with error 1920 with a start date of 24th July, and an end date of 31st August.
However, the last school day in the academic calendar is the 21st July. Because of this, the student's leaving date is output in the census as the 21st July.
The DfE wouldn't usually expect students to have a start date after the end of term, so you'll need to amend their enrolment dates. For further guidance, please contact the DfE.