Load key "privkey.ppk" invalid format

What have I done:
I'm trying to connect to my remote server with private key with

ssh -i privkey.ppk root@ip

But it's returning an error.
Error message:

Load key "privkey.ppk": invalid format
root@ip: Permission denied (publickey).

Expected result:
I should be able to login into my remote server with ssh key.

Key file content:

PuTTY-User-Key-File-2: ssh-rsa
Encryption: aes256-cbc
Comment: rsa-key-20181019
Public-Lines: 6
some lines
Private-Lines: 14
some lines
Private-MAC: some letters

OS: Ubuntu 18.04

(p.s:

1. I'm using the same key in putty, termius to login into my remote server and its working fine
2. Ansible ping cmd also returning the same result)

This key is generated by PuTTYgen software. To use it in linux you should start PuTTYgen, load the key (File->Load private key) and then export it as OpenSSH key ( Conversions->Export OpenSSH key)

My PPK was in putty PPK format I had to convert into OpenSSH format.
I converted .ppk to OpenSSH using this answer, now I'm able to connect.

puttygen id_dsa.ppk -O public-openssh -o id_dsa.pub