There are a few potential reasons why the "Failed to checkout the repository" error might be occurring in your scenario. Here are a few things to check:

1. Permissions: Make sure that the user running the Odoo instance has the necessary permissions to access the submodule repository.

2. Network connectivity: Ensure that the Odoo instance has access to the submodule repository over the network. You can try running a "ping" test to see if the server can reach the repository.

3. Branch availability: Make sure that the branch "test_branch" is still available in the submodule repository and that the commit you are trying to check out exists on that branch.

4. Correct URL: Ensure that the URL for the submodule repository is correct and that it's reachable.

5. Git version: Make sure that the version of Git being used by the Odoo instance is compatible with the version of Git used by the submodule repository.

6. Credentials: Make sure that the correct credentials are being used to access the submodule repository.

7. Updating odoo sh: You can try updating the Odoo SH to the latest version to see if the issue is resolved.

If none of the above suggestions solve the problem, you may want to look at the Odoo server logs and the git command output to see if they provide any additional information about the error. Additionally, you can also reach out to Odoo support for further assistance.