Clangd-9 project indexation.

Hello. Recently I’ve become interested in the mlpack C++ library ( I’ve cloned the repository, ran CMake with the option to generate compile_commands.json and tried opening the project in emacs with lsp-mode enabled (clangd-9.0.0-2). During indexing, there were a lot of messages in the clangd::stderr buffer like:
Failed to compile /home/username/mlpack/src/mlpack/tests/linear_regression_test.cpp, index may be incomplete
Then, after clangd finished indexing, I could not find many definitions and many parts of the source code were highlighted red and there were error messages like
clang: No template named 'vector' in namespace 'std'; did you mean 'std::std::vector'? (Fix available)
clang: Use of undeclared identifier 'arma'; did you mean 'std::arma'? (Fix available)
(Where 'arma' is a namespace declared in armadillo, a dependency of mlpack)

Do you have any idea on what is wrong? Thank you in advance.