PuTTY's private SSH key can't be used interchangeably with OpenSSH clients because they both use and support a different key format.
You need to convert PuTTY's key file, which uses the .ppk extension and stands for PuTTY Private Key, to the OpenSSH Private Key Format before you can use it with OpenSSH.
PuTTYGen or PuTTY Key Generator is a tool you can use to convert the PuTTY Private Key (.ppk) file to the OpenSSH private key format.
There are an old and new types of SSH key file formats and will be automatically be determined based on the key's type except if you choose Export OpenSSH key (force new file format).
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