Error accessing submodule

I get the following error when I try to get the submodule. Any ideas what the problem is?

git submodule update --init

Gives the following error

Please make sure you have the correct access rights
and the repository exists.
fatal: clone of 'git@github.com:mathause/cmip_warming_levels.git' into submodule path '/g/data/xv83/rxm599/acs/gwls/cmip_warming_levels' failed
Failed to clone 'cmip_warming_levels'. Retry scheduled
Cloning into '/g/data/xv83/rxm599/acs/gwls/cmip_warming_levels'...
The authenticity of host 'github.com (20.248.137.48)' can't be established.
ED25519 key fingerprint is SHA256:+DiY3wvvV6TuJJhbpZisF/zLDA0zPMSvHdkr4UvCOqU.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Failed to add the host to the list of known hosts (/home/599/rxm599/.ssh/known_hosts).
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Begin with the `git submodule update --init` command, the `cmip_warming_levels` submodule, and the reported SSH clone output. Determine why this environment cannot clone the submodule and record a reproducible resolution or required repository-access change; the issue names no source file or test.

Written by the indexing model from the issue text.