Every record has a priority. The -p option keeps records of the severity you need — see what remains of the two lab-demo runs after such a selection.
The journal with an err threshold
Print the lab-demo.service records after the timestamp in /srv/log-lab/since with an err priority threshold. Compare them with the full journal from the start of the lesson.
First, predict
What will remain of the first, failed run in the -p err output?
Now start the exercise, run the command and compare your prediction with the output.