Hmm… error msg makes me suspect that the cmd is expecting me to be using a public key to authenticate with GitHub, but I use a password: BTW,
$ git remote add upstream git@github.com:TEIC/TEI.git
$ git pull upstream dev
The authenticity of host 'github.com (140.82.114.4)' can't be established.
RSA key fingerprint is SHA256:nThbg6kXUpJWGl7E1IGOCspRomTxdCARLviKw6E5SY8.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes

Warning: Permanently added 'github.com,140.82.114.4' (RSA) to the list of known hosts.
git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.
BTW, did you actually mean the username “git”? Just in case I tried with “sydb”, too; same error.

git remote add upstream git@github.com:TEIC/TEI.git (if you haven’t already done this)
git pull upstream dev

ought to do the trick.