error: It was not possible to parse your key

simple oauth version: 8.x-4.5
OpenSSL version: 1.0.2g 1 Mar 2016

following the instruction https://medium.com/@nikolay.r.borisov/drupal-8-simple-oauth-and-simple-r...
use postman to get the token, got the following error:

InvalidArgumentException: It was not possible to parse your key, reason: error:0906D06C:PEM routines:PEM_read_bio:no start line in Lcobucci\JWT\Signer\OpenSSL->validateKey() (line 90 of /mnt/www/html/olamaindev/vendor/lcobucci/jwt/src/Signer/OpenSSL.php).
Lcobucci\JWT\Signer\OpenSSL->getPrivateKey('', NULL) (Line: 21)
Lcobucci\JWT\Signer\OpenSSL->createHash('eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImp0aSI6ImNmYzBiYTVmNGM3YzUzYTBkNzEzYzE4NTM3OTE3NjRhYjBlYjhmYzVjMTI4NWJmYmZkNjAxZDgyNzNiYjlkMWQ5MjRlMDVjZGQ0ZWY4YWJiIn0.eyJhdWQiOiIxMDUwYjJjYy04NDM5LTRlNDItOTNjNy1jNzY2YTI5NjUzNWIiLCJqdGkiOiJjZmMwYmE1ZjRjN2M1M2EwZDcxM2MxODUzNzkxNzY0YWIwZWI4ZmM1YzEyODViZmJmZDYwMWQ4MjczYmI5ZDFkOTI0ZTA1Y2RkNGVmOGFiYiIsImlhdCI6MTU5ODIzNjkwMSwibmJmIjoxNTk4MjM2OTAxLCJleHAiOjE1OTgyNjY5MDEsInN1YiI6IjE0MTUyMSIsInNjb3BlcyI6WyJhdXRoZW50aWNhdGVkIiwiZGV2ZWxvcGVyIiwiZWRpdG9yIl19', Object) (Line: 36)
Lcobucci\JWT\Signer\BaseSigner->sign('eyJ0eXAiOiJKV1QiLCJhbGciOiJSUzI1NiIsImp0aSI6ImNmYzBiYTVmNGM3YzUzYTBkNzEzYzE4NTM3OTE3NjRhYjBlYjhmYzVjMTI4NWJmYmZkNjAxZDgyNzNiYjlkMWQ5MjRlMDVjZGQ0ZWY4YWJiIn0.eyJhdWQiOiIxMDUwYjJjYy04NDM5LTRlNDItOTNjNy1jNzY2YTI5NjUzNWIiLCJqdGkiOiJjZmMwYmE1ZjRjN2M1M2EwZDcxM2MxODUzNzkxNzY0YWIwZWI4ZmM1YzEyODViZmJmZDYwMWQ4MjczYmI5ZDFkOTI0ZTA1Y2RkNGVmOGFiYiIsImlhdCI6MTU5ODIzNjkwMSwibmJmIjoxNTk4MjM2OTAxLCJleHAiOjE1OTgyNjY5MDEsInN1YiI6IjE0MTUyMSIsInNjb3BlcyI6WyJhdXRoZW50aWNhdGVkIiwiZGV2ZWxvcGVyIiwiZWRpdG9yIl19', Object) (Line: 470)
Lcobucci\JWT\Builder->createSignature(Array, Object, Object) (Line: 450)
Lcobucci\JWT\Builder->getToken() (Line: 44)
Drupal\simple_oauth\Entities\AccessTokenEntity->convertToJWT(Object) (Line: 28)
League\OAuth2\Server\ResponseTypes\BearerTokenResponse->generateHttpResponse(Object) (Line: 202)
League\OAuth2\Server\AuthorizationServer->respondToAccessTokenRequest(Object, Object) (Line: 87)
Drupal\simple_oauth\Controller\Oauth2Token->handleToken(Object, Object) (Line: 63)
Drupal\simple_oauth\Controller\Oauth2Token->token(Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 573)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 151)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 68)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 67)
Drupal\simple_oauth\HttpMiddleware\BasicAuthSwap->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 59)
Drupal\lao_url_redirects\LaoUrlRedirectsStaticPageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 52)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

but in "simple Oauth Settings" page under "token" tag, I saw that there are access token and refresh token been created.

any suggestions?