No private key available settings : SAML SSO Nextcloud 16

Nextcloud version: 16
System: Debian
Identity Provider: WSO2 identity server
I Configure SSO & SAML with Nextcloud. Everythings look fine. But when I log in Nextcloud through my WSO2 identity Server login page, I have an error: "The server was unable to execute this request ". When I see the log file, it says:

{“reqId”:“FKGBbOOpf9f0RPV68oxi”,“level”:3,“time”:“2020-02-19T18:33:46+00:00”,“remoteAddr”:“”,“user”:"–",“app”:“index”,“method”:“POST”,“url”:"/index.php/apps/user_saml/saml/acs",“message”:{“Exception”:“OneLogin\Saml2\Error”,“Message”:“No private key available, check settings”,“Code”:13,“Trace”:[{“file”:"/var/www/html/nextcloud/apps/user_saml/3rdparty/vendor/onelogin/php-saml/src/Saml2/Response.php",“line”:114,“function”:“decryptAssertion”,“class”:“OneLogin\Saml2\Response”,“type”:"->",“args”:["*** sensitive parameters replaced ***"]}

On Nextcloud SSO SAML interface, I don’t where i can enter the private certificate key. There is just only: Public certificate key field. Help me solve this issue