Your LTHR is the average of your final 15 minutes of that 20-minutes test. The Garmin site does not allow me to filter by that 15 minutes, but I would guess that it was 181 for the last 15 minutes, therefore enter in 181 on the LTHR field. Next time, just hit the lap button 5 minutes into the test and then you’ll have your peak 15-minutes lap we can use.
For pace, select the 20-minute pace method and enter in your average pace, which was 5:50. This results in a threshold pace of 6:08 per mile.