When i install the demo, i had a error: d3_dx9.dll or something like that. I use a DirectX 11 (!. Help!

You need to install DirectX 9, because LFS required this API Version.
https://www.microsoft.com/en-US/download/details.aspx?id=34429

whenever i type in my online details it works, however the screen then goes black but everything still works and when i find a mode (eventually)that allows me to go on the track i can see again then i exit the race and everything is normal. PLEASE HELP

Update graphics drivers, update DirectX, update Visual C++ and update .Net to 4.6.2

That always works for me