I recently bought this game and after selecting "launch game" you see this message: "The code cannot be continued because the d3dx9_35.dll object was not found. Reinstalling the program may solve this problem." But reinstalling doesn't help and I still can't start the game. What can I do?

This error appears when your computer is missimg DirectX 9. You can download it at Microsoft's website:
https://www.microsoft.com/en-us/download/details.aspx?id=35&44F86079-8679-400C-BFF2-9CA5F2BCBDFC=1

I recently took The Witcher Enhanced Edition on GOG.com I tried to launch the game via GOG Galaxy and i don't know why the game simply won't launch can someone help me? i was really looking forward to enter in the Gerald's triology!

Same thing over here. The game doesnt even launch. I have no idea why it wouldnt. The game should be compatible with my pc. i am going to try to download the latest version of directX.

You don't need the latest version.
You need DirectX 9.0c that the game uses.
Go here https://www.microsoft.com/en-us/download/details.aspx?id=8109
Download the runtimes to a temporary folder on your desktop & run DXSETUP.exe
Once it says installation complete (see pic) you can delete the folder from your desktop.
And don't worry, it won't effect your newer DirectX files.

the link is not available... this is a problem...

In this thread some users came up with alternate sources to download the DirectX files: https://www.gog.com/forum/general/microsoft_has_removed_the_directx_9_installers_from_their_download_site/page1