Did not sleep is when for some reason your Mac stayed awake when it should have fallen asleep.
There are 3 common causes of "Not Sleeping".
To solve this issue, we need to try to identify what the issue is, we do that by selecting a "Did Not Sleep" event in the Sleep Aid history window.
Software can tell the macOS, not to go to sleep, if say it's downloading a large file, processing a document, awaiting a command from a server, playing Music or video. "Potential Software causes" will list apps and Apple services that have requested the Mac to stay awake.
If " Music" is the cause, you can enable "Pause Music during sleep" in the "Sleep Settings" of Sleep Aid.
Otherwise, pausing music or video playback should be enough, if not, you may have to quit the application when you don't need it.
Settings displayed in this list are potential reasons for preventing sleep, they can be disabled in the "Sleep Settings" window of Sleep Aid. Make sure that "System Sleep Timer" is set to 1.
The last option to figure out if it is an external device that's keeping the Mac awake. Start by unplugging the devices and seeing if the Mac will then sleep. Sleep Aid will keep a record of the connected devices, as this will help to determine which one (if any) was responsible.