According to Tiki requirements, the application only supports PHP 5.6. You are trying to install it on top of XAMPP 7.1.4 (PHP 7.1.4), so the issue is normal because it is an unsupported version of PHP.
You can try to install Tiki on top of XAMP 5.6.30 (PHP 5.6.30).
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.
Carlos R. Hernández