fatal: not a valid object name: 'master'

This error message typically occurs when trying to reference a Git branch that does not exist. In this case, the branch "master" may not have been created in the repository or may have been deleted.

To resolve the issue, you can try the following steps:
1. Verify that the branch "master" exists by running `git branch` command in your terminal.
2. If the branch does not exist, you can create it using `git branch master` command.
3. If the branch was deleted, you can try to restore it from a previous commit by checking out the commit where the branch existed and then creating a new branch from that commit using `git checkout <commit SHA>` and then `git branch master`.

If none of the above steps work, you may need to seek further assistance or review the Git log to determine if there were any recent changes to the repository that caused the issue.