WindowServer prevents sleep

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Does the issue present in this mode ?

3 - Sometimes a Safe Boot followed by a Normal Boot will just put things right.

1 - There could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

2 - If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

3 - If after performing each of the above steps is the order that have been presents  and still have issues - please advise for possible further assistance

Writing an effective Apple Support Communities question

Is this a New Issue ? If Yes, what exactly changed on this computer ?

Is this an Old Issue ? If Yes, what actions has the user taken to fix this issue ?

After all,  we do not what to suggest possible solutions for what has already been attempted by the User 

Understand, we are all working Remotely from all around the world. 

Therefore, we do not have the On-Hands experience the User ( you ) has with this computer.

Reliable , accurate, details and context will really help to pinpoint a possible solution.