Fatal error: Uncaught Error: Undefined constant "MB_OVERLOAD_STRING" in /volume1/web/swwagro/libraries/vendor/joomla/string/src/phputf8/utf8.php on line 38

Error: Undefined constant "MB_OVERLOAD_STRING" in /volume1/web/swwagro/libraries/vendor/joomla/string/src/phputf8/utf8.php on line 38

Call Stack:
    0.5605     355232   1. {main}() /volume1/web/swwagro/index.php:0
    0.7162     357360   2. require_once('/volume1/web/swwagro/includes/framework.php') /volume1/web/swwagro/index.php:40
    0.9923     361464   3. require_once('/volume1/web/swwagro/libraries/cms.php') /volume1/web/swwagro/includes/framework.php:18
    1.0088     362752   4. require('/volume1/web/swwagro/libraries/vendor/autoload.php') /volume1/web/swwagro/libraries/cms.php:36
    1.0260     362944   5. ComposerAutoloaderInit205c915b9c7d3e718e7c95793ee67ffe::getLoader() /volume1/web/swwagro/libraries/vendor/autoload.php:7
    1.1131     364240   6. composerRequire205c915b9c7d3e718e7c95793ee67ffe($fileIdentifier = '2fb9d6f23c8e8faefc193a4cde0cab4f', $file = '/volume1/web/swwagro/libraries/vendor/composer/../joomla/string/src/phputf8/utf8.php') /volume1/web/swwagro/libraries/vendor/composer/autoload_real.php:56
    1.1487     364848   7. require('/volume1/web/swwagro/libraries/vendor/joomla/string/src/phputf8/utf8.php') /volume1/web/swwagro/libraries/vendor/composer/autoload_real.php:66