What should I assign to ECOMMERCE_GIT_IDENTITY?

I’m trying to set our git repository address in server-vars.yml file. though each value I set for ECOMMERCE_GIT_IDENTITY it ends up with the following error:

Permission denied (publickey,keyboard-interactive).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights and the repository exists.

I have a private key with which I can clone the repository manually. but It seems to not work when I assign its content to ECOMMERCE_GIT_IDENTITY.
could you guide me, please?