When I have my project finished and do the render selecting Tehora o webm and play at the end and just get a black screen that says invalid.

Any help you could give me.

kubuntu 12.04

So your rendered video just shows the word "INVALID" on black background when you play it in some video player?

Maybe there is an issue with one of the clips in your project - I think kdenlive "renders" such a clip as "INVALID" on black background, but I am not sure, it should show the same already in the monitor.