I think the clang is not correctly installed on the ubuntu box.
Please try to Install clang by following commands:
sudo apt-get install clang
clang -v //Please check the installed version
Now your error must be fixed.
I also had faced the same problem in recent past and also solved it with the below steps:
1. I had firstly installed the llvm plus clang which is available on the following link http://releases.llvm.org/
2. After that I had installed the C++ language adapter which is available on the following link https://marketplace.visualstudio.com/items?itemName=mitaki28.vscode-clang
3. Lastly I configured the path of the llvm/bin folder installed in Step 1 above and in the visual studios code that has the 'user settings', with the help of clang.executable: variable. After that please refer to the Step 2 from 'configuration' section explained in the above url.