How to resolve “The key you are authenticating with has been marked as read only”?

I did the following:

$ ... create new file ...
$ git add *
$ git commit -m "first-file"
$ git push origin main
...
The key you are authenticating with has been marked as read only.
fatal: could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists

Why am I getting this error and how do I fix the problem?

The SSH key you've added is a deploy key, which is a key that's associated with a repository and can be either read-only or read-write. Normally, most SSH keys are associated with a user, and access repositories based on the permissions of that user (possibly with limitations related to SSO).

However, deploy keys are specifically designed to be associated with a single repository, and they're a good way to allow automated services to work with repositories without having to worry about what happens when a user loses access.

In your case, the deploy key you're using has been marked read-only, and so it cannot be used to push. If you had intended this account to be associated with your account instead of the repository, go into the repository settings and remove it, and then go to your account settings and add it there. If you did intend for it to be a deploy key, then go into repository settings, delete the key, and re-add it, checking the "Allow write access" option.

Note that a single key can be only one of a read-only key for a single repo, a read-write key for a single repo, or a user key. Because GitHub identifies access based on the key, you cannot use the same SSH key for multiple purposes.