How can I install OpenMP in Ubuntu?

You don't install OpenMP. OpenMP is a feature of the compiler. Check that the compiler you are using implements OpenMP, which is an API for parallel programming in C/C++/Fortran.

Check with the OpenMP website, openmp.org for a list of compilers and platforms that support OpenMP programming.

sudo apt install libomp-dev

apt show libomp-dev

Description: LLVM OpenMP runtime - dev package The runtime is the part of the OpenMP implementation that your code is linked against, and that manages the multiple threads in an OpenMP program while it is executing.