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 version is not supported.
To resolve the problem, install the latest available PHP version in Tools & Settings > Updates and Upgrades.
Required PHP extension is turned off.
To resolve the problem, turn on the required extension. You can do this by adding a certain PHP directive to the server-wide
php.inifile 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.
After you add the directive, restart a web server. Learn how to do this on Linux and on Windows.