I have been fighting all day with this issue, and although in the end I have not found the solution, I have found a workaround with which we can complete the installation without errors.
Apparently this error only occurs when using XAMPP in Windows environment.
Before starting the installation process, you have to edit the “php.ini” file and set the following value “display_errors = Off”. Then restart the Apache server and you can start the installation. Once the installation is completed, you can reset the parameter as it was and restart the Apache server again.
@JoomShaper Team: Please include this workaround in the documentation of the HelixUltimate (and probably many of the templates) installation process, as I've seen it recurring for years now, and I hope this hint can help you figure out where the problem is.