You are being asked for the passphrase for your private key, not for the password to your account. Normally, the file containing a private key is encrypted with a passphrase, so that if someone gains access to your hard disk or to your backups, they won't be able to use your key. You need to enter the passphrase to decrypt the private key. If you aren't worried about losing your key in this way, you can set an empty passphrase, so that the private key is left unencrypted.
.ppk is the default extension for PuTTY's private key files; OpenSSH key files normally have the extension
.id_ecdsa. OpenSSH and PuTTY keys are compatible, but they use different file formats. If you have a PuTTY private key file, you need to export it to OpenSSH format with the PuTTYgen utility.