Fatal error after autorization

I called the first endpoint : /wp-json/openid-connect/v1/authorize?response_type=code&client_id=postman&scope=openid profile&state=af0ifjsldkj

and it sent me to the authorization screen, clicking on authorize I get this error.

Please can you help me understand what went wrong?

Warning: openssl_sign(): Supplied key param cannot be coerced into a private key in?/var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Encryption/Jwt.php?on line?161

Fatal error: Uncaught Exception: Unable to sign data. in /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Encryption/Jwt.php:162 Stack trace: #0 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Encryption/Jwt.php(139): OAuth2\Encryption\Jwt->generateRSASignature() #1 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Encryption/Jwt.php(31): OAuth2\Encryption\Jwt->sign() #2 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/OpenID/ResponseType/IdToken.php(147): OAuth2\Encryption\Jwt->encode() #3 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/OpenID/ResponseType/IdToken.php(118): OAuth2\OpenID\ResponseType\IdToken->encodeToken() #4 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/OpenID/Controller/AuthorizeController.php(62): OAuth2\OpenID\ResponseType\IdToken->createIdToken() #5 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Controller/AuthorizeController.php(135): OAuth2\OpenID\Controller\AuthorizeController->buildAuthorizeParameters() #6 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Server.php(383): OAuth2\Controller\AuthorizeController->handleAuthorizeRequest() #7 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/src/Http/Handlers/AuthorizeHandler.php(56): OAuth2\Server->handleAuthorizeRequest() #8 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/src/Http/Router.php(117): OpenIDConnectServer\Http\Handlers\AuthorizeHandler->handle() #9 /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/src/Http/Router.php(111): OpenIDConnectServer\Http\Router->do_handle_request() #10 /var/www/fir/httpdocs/wp-includes/rest-api/class-wp-rest-server.php(1230): OpenIDConnectServer\Http\Router->handle_rest_request() #11 /var/www/fir/httpdocs/wp-includes/rest-api/class-wp-rest-server.php(1063): WP_REST_Server->respond_to_request() #12 /var/www/fir/httpdocs/wp-includes/rest-api/class-wp-rest-server.php(439): WP_REST_Server->dispatch() #13 /var/www/fir/httpdocs/wp-includes/rest-api.php(428): WP_REST_Server->serve_request() #14 /var/www/fir/httpdocs/wp-includes/class-wp-hook.php(324): rest_api_loaded() #15 /var/www/fir/httpdocs/wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters() #16 /var/www/fir/httpdocs/wp-includes/plugin.php(565): WP_Hook->do_action() #17 /var/www/fir/httpdocs/wp-includes/class-wp.php(418): do_action_ref_array() #18 /var/www/fir/httpdocs/wp-includes/class-wp.php(813): WP->parse_request() #19 /var/www/fir/httpdocs/wp-includes/functions.php(1336): WP->main() #20 /var/www/fir/httpdocs/wp-blog-header.php(16): wp() #21 /var/www/fir/httpdocs/index.php(17): require(‘…’) #22 {main} thrown in /var/www/fir/httpdocs/wp-content/plugins/openid-connect-server/vendor/bshaffer/oauth2-server-php/src/OAuth2/Encryption/Jwt.php on line 162