So I just moved over to 4.2 and was importing some models a total of 4 so far. Very simple Models, they are letters. for 2 of the 4 when I click on Generate Unique UV’s I get the following error. That’s so far is a 50% failure rate and has me worried.
Could Convert to a D3DXMesh
I have never seen this error before so is it a 4.2 bug or something else. Its such a simple model as well.
It’s not a bug from the 4.2, this sometimes even happens in UDK 
Take a look at this thread, probably you will find a solution in it: https://forums.epicgames/threads/713367-GenerateUV-s-failed-couldn-t-convert-to-a-D3DXMesh
**Easiest solution: **generate your lightmap in your 3d program ->
Um that thread doesn’t provide much of a solution and I do not even know if that settings exists in this engine or what changing it would do. I am hesitant to mess with that.
Look I just tested some more models, now before I moved to 4.2 I have imported and generated Unique UV’s 100’s of times. 99.5% of all our assets would do this without issue. I just imported a dozen small simple models and 50% of them will not generate. Yes I know I could do it in another program but that takes up more time. Also it was something that was working before without an issue.
So your solution is um well I am just gonna say “odd”. If I told you I was going to remove all the landscape and foliage tools and geometry tools from UE4 because you can do it in another program which you could, Just doesn’t seem like a good solution. It would be more like going backwards in progress than forwards.
So being that this just cropped up for me in 4.2 I figure I would report it and see if it makes it into a hotfix. I will try to get a few more of my fellow devs to try this out in 4.2 and see if they have the same issues.
Please don’t take it the wrong way - you help out everyone a ton.
Probably this just happens in your project -> you could send me one of those meshes and I can try it out in my UE4
But in all those years that I used one of the unreal engines I got this error several times, but now solution was found (except of generating the lightmaps in a 3d program -> in blender this just takes around 10 seconds ^^) -> during the beta time I have even sent one of my meshes to epic games, but haven’t got a reply 
Well I dunno what was going on. I have been busy so I haven’t had a chance to focus on the above and test it all till now. However I just wrapped up that little project and went through all the modeled letter about a dozen of them and they all generate UV’s now without issue.
Maybe its was because it was a fresh upgrade or a reboot or something I am not sure. I just proceeded ahead working on it not worrying about UV’s because I knew I could come back and fix that later if needed. But now that the scene is finalized I was going through to make it all finished and they all compiled UV now.
So I do not know what was going on… Thanks for your involvement though
How do you do that on blender?
Here we go -> first of all export the mesh from the UE4 -> import it -> follow the tutorial -> re-import it