Here is a detailed guide on how to solve this problem, if you have windows 10 and windows defender is tagging the game client as a virus, a false positive, here is how to solve it:
Step 0 - Notification (Optional)
Allow the file as pictured once you see a virus report notification and click on it
Step 1 - Add a folder exception to your game folder
Windows Defender -> Virus & Threat Protection -> Virus & Threat Protection Settings -> Exclusions
Your default game folder location should be c:\Users\Your User\AppData\Local\ There select the game folder Celestia Luna Online and add it to exclusions.
Step 2 - Turn off real time protection
Same location Windows Defender -> Virus & Threat Protection -> Virus & Threat Protection Settings
Step 3 - Reinstall
Now it's time to reinstall the game, because of how windows defender works it won't restore the file even if you try.
After reinstalling you must patch fully before continuing to the next step!
Step 3 - Add a file exception
Located in your game folder you will find a file called LUNAClient.exe, add this file to exceptions.
Now you should have the following exceptions listed:
Final Step - Re-enable real time protection
And lastly enable real time protection back on and enjoy playing!