When I set up a docksal project, it uses git and the project folder ends up with a remote:
https://github.com/docksal/boilerplate-drupal9-composer.git
I then add another remote, to the repo for the codebase I am developing.
The problem is that the clone done by docksal was shallow, and when I try to push to my repo the push is rejected because of the shallow setting (the shallow file in .git)
Feel free to drop the .git folder and initialize a fresh git repo in the project folder Docksal created.
In case you'd prefer to inherit the boilerplate git repo commit history, you can always clone the repo directly:
git clone https://github.com/docksal/boilerplate-drupal9-composer.git drupal9
cd drupal9
fin project start