Fatal error: Declaration of PDOStatement::fetch(int $mode = PDO::FETCH_BOTH, int $cursorOrientation = PDO::FETCH_ORI_NEXT, int $cursorOffset = 0) must be compatible with Gibbon\Contracts\Database\Result::fetch($fetch_style = null, $cursor_orientation = null, $cursor_offset = null) in C:\xampp\htdocs\gibbon\src\Database\Result.php on line 0
It sounds like you may be using PHP 8, which is not supported yet in Gibbon v21. Your best bet is to ensure your server uses a PHP version between 7.3 and 7.4.
We have been working on PHP 8 support in the v22 development branch. This version isn’t released yet, but if you’re testing Gibbon rather than using it in production, you can check out the v22 repo on GitHub: https://github.com/GibbonEdu/core