In your project parent directory, run:
git submodule update --init
Or if you have recursive submodules run:
git submodule update --init --recursive
Sometimes this still doesn't work, because somehow you have local changes in the local submodule directory while the submodule is being updated.
Most of the time the local change might not be the one you want to commit. It can happen due to a file deletion in your submodule, etc. If so, do a reset in your local submodule directory and in your project parent directory, run again:
git submodule update --init --recursive