I can connect my vps ip with putty ,hostname is a.b.c.d , ftp user is root ,password is password . None of connection type (ftp or ftps) can add a new plugin .
Failed to connect to FTP Server
To perform the requested action, WordPress needs to access your web server. Please enter your FTP credentials to proceed. If you do not remember your credentials, you should contact your web host.
These answers are provided by our Community. If you find them useful, show some love by clicking the heart. If you run into issues leave a comment, or add your own answer to help others.