The WP Cloud plugins use a cache which is stored in the /wp-content/[WP Cloud Plugin name]-cache/ folder. Only when this folder exists and is writable by the plugin, the plugin can be used.
This error is thrown when the plugin isn't able to create the cache folder or when it is (no longer) writable. Normally, this should automatically be solved via the plugin but it is possible that the file/folder permissions are not set to the default WordPress values on your server preventing it to change the permissions.
Solution
Navigate to the /wp-content/[WP Cloud Plugin name]-cache/ folder via (s)FTP and update the folder permissions to 755
If the permission of this folder is already set to 755, please try to manually copy the contents of the /wp-content/plugins/[WP Cloud Plugin name]/cache folder into the /wp-content/[WP Cloud Plugin name]-cache/.
If this doesn't help you, please create a support ticket and provide your purchase code and a temporarily WP admin login + temporarily (s) FTP login so we can safely debug the issue for you.