Load key "/usr/keys/key.ppk" : invalid format

[j143]

Load key "C:\\Users\\user\\Downloads\\keys\\aws\\dev-server\\dev-server.ppk":
>  invalid format
> 
[08:24:25.465] > ubuntu@12.3.4.204: Permission denied (publickey).
> 
[08:24:25.477] > The process tried to write to a nonexistent pi

Solution:

Use .pem format for the key