The submodules option is used when you are using private modules as submodules. This setting is only needed for private submodules; public submodules will work fine, without any issues. It is not possible to download private repositories publicly, so you need to give repository access to Odoo.sh. Follow these steps to add access to the private submodules:

1. Copy the SSH URL of your private submodule repository in the input and click on Add.
2. Copy the displayed Public Key.
3. Add this public key as a deploy key in your private repository settings in GitHub (similar settings are also available on bitbucket and GitLab):