Your game download was most likely corrupted. We suggest using a download manager to help in these situations such as Free Download Manager.
You are missing .NET Framework 4.0, which doesn't come with old operating systems such as XP. We strongly advice to upgrade your machine's operating system if you are still using Windows XP as it is no longer supported by Microsoft and is the most vulnerable operating system.
An is provided within your game folder located in "C:/Users/Your User/AppData/Local/Celestia Luna Online/" by default.
If you are encountering that you already have that version of .NET Framework installed that means your .NET Framework has been corrupted.
A great amount of games require different versions of .NET Framework to work.
To try and fix this problem you may download a repair tool by Microsoft:
Microsoft .NET Framework Repair Tool | Microsoft .NET Framework Repair Tool Guide
The solution can be found in point 4.
Alternatively you may follow our guide: Unable to find client executable solution guide.
Some antivirus programs flag our game client as a false positive because of our anti-cheat and block or quarantine it.
You can play with your antivirus disabled or allow the game client to its exceptions list, which is a safer option.
If you don't see "LUNAClient.exe" in your game folder that means your antivirus quarantined it and you must restore it before adding it to exceptions.
If this is still happening after adding the game to your antivirus' exceptions you are missing a VC Redist installation. Download and install it directly from Microsoft:
VC Redist x86 | VC Redist x64