When an app cannot be installed to a customer's website due to some reason, a customer gets an error message with the recommendation to contact their hosting provider (you). The error message also contains the brief error description that should help you find the reason of a problem.
The most common problem that may occur during an app installation is when PHP does not meet app requirements:
php.ini
file or to the custom PHP configuration of the subscription (in case you want the extension to be available to a certain user only). Learn how to do this in the section Customize PHP Configuration.