YouCompleteMe requires vim to have support for python scripting. Run the following and ensure that there is a + before python3:
vim --version | grep python
If not, you will need to compile vim from source (don't worry, it's easy).
YouCompleteMe depends on some programs being installed to build successfully; see the (short) installation guide. For Ubuntu 20.04+ for example:
sudo apt install build-essential cmake python3-dev
Vim-plug will have successfully built YouCompleteMe if you already had these
dependencies installed when vim-plug downloaded YouCompleteMe for the first
time. Otherwise, run :PlugInstall! in vim to force a rebuild of
YouCompleteMe.
Following this short guide should be enough to successfully build a basic build of YouCompleteMe. If you're on a different distro than the ones listed or if something goes wrong with the installation, refer to the full installation guide.
The short installation guide also includes instructions on how to enable semantic completion for supported languages and their respective dependencies (e.g. C/C++ semantic completion requires libclang, and Go requires, well, Go to be installed).
Note that you don't need semantic completion, but it can be nice to have especially if the set-up doesn't take much effort (e.g. Go).
Rebuilding YouCompleteMe with the appropriate flags should be enough to enable semantic completion for your target languages. However, to maximize your experience, read the user guide for language-specific set-up instructions.