When working with a remote access PuTTY client in Windows, an error may occur due to incompatibility of key formats:
Unable to use key file (OpenSSH SSH-2 private key (old PEM format))
To eliminate this error, the key must be converted by the PuTTYgen tool. Download the utility through the website, open it, and select the "Conversions" tab in the top panel. In the menu that opens, press "Import key" and select the key.
To convert Keys after import, click "Save private key." The key will be saved in a new format (.ppk) with the selected name. If necessary, you can also set the password for the key.