This error indicates that the server requires public key authentication and no key was provided by the client.
TortoiseGit by default uses TortoiseGitPLink (which is based on Putty Link).
Therefore it requires key to be stored in the putty format and also
- configured in the putty session (cf. https://tortoisegit.org/docs/tortoisegit/tgit-ssh-howto.html) OR
- loaded in putty agent (this is done by TortoiseGit if loading ssh keys is enabled; can also be done manually by starting Putty authentication agent (can be found in the Startmenu or in TortoiseGit installation folder) and then loading your key; also other alternatives such as KeePass KeyAgent integrations exist).
You can, however, also use OpenSSH, by setting the SSH client to ssh.exe
on TortoiseGit -> Settings -> Network if you have a OpenSSH key configured.