Compile Error

Hi, I try to work with the sdk sample, but if given some errors like this:

Getting access token...
Warning: openssl_sign(): supplied key param cannot be coerced into a private key in /var/www/..../include/adobe_stock_sample/vendor/firebase/php-jwt/src/JWT.php on line 196

Fatal error: Uncaught DomainException: OpenSSL unable to sign data in /var/www/...../include/adobe_stock_sample/vendor/firebase/php-jwt/src/JWT.php:198 Stack trace: #0 /var/www/...../include/adobe_stock_sample/vendor/firebase/php-jwt/src/JWT.php(167): Firebase\JWT\JWT::sign('eyJ0eXAiOiJKV1Q...', '-----BEGIN RSA ...', 'RS256') #1 /var/www/.../include/adobe_stock_sample/src/Utils/ImsToken.php(59): Firebase\JWT\JWT::encode(Array, '-----BEGIN RSA ...', 'RS256') #2 /var/www/....../include/adobe_stock_sample/src/sdk_license1.php(39): SdkDemo\Utils\ImsToken->__construct(Object(SdkDemo\Config\AppConfig)) #3 /var/www/..../fotodruck_konfigurator.php(19): include_once('/var/www/.....') #4 /var/www/....../index.php(2899): include_once('/var/www/.....') #5 {main} thrown in /var/www/...../include/adobe_stock_sample/vendor/firebase/php-jwt/src/JWT.php on line 198