Did you maybe reinstall your operating system or League of Legends and now the client runs fine but you can't get into a game because an error pops up saying d3dx9_39.dll is missing? A fix for this issue is unusually hard to find on the internet but it's a matter of simply getting a proper DirectX download.
How to fix League of Legends "d3dx9_39.dll missing" error
The fix is as simple as downloading a DirectX package from the official Microsoft website . This is an executable file which may raise an eyebrow but even your PC will state it's from Microsoft when you try to run it so don't worry, there is no malware involved.
Make sure you run League of Legends as an Administrator. You can toggle this by right-clicking the shortcut, going to Properties and switching to Compatibility card. Tick the box next to "Run this program as an administrator" and you're good to go.
Searching for "d3dx9_39.dll missing" error fix can take you far and wide over the internet, as it has done to me in the past, but this particular way proved to be extremely fast and doesn't require visiting fishy links or downloads. The reason why you usually don't see this fix in the top results is that it was hiding on Reddit , with no upvotes.
Technically, Riot Games pointed out the same way to fix the error but provided a link to Microsoft's website instead of the direct link to DirectX download, which could prove confusing to some players.
In case this method didn't work for you, there are some alternatives such as downloading the .dll and manually placing it in System32 but that one is risky and I have not tried doing it yet. Therefore, I cannot recommend that course of action.