After the installation or upgrade, it may happen that this screen would appear and as a result, merchant, and affiliates would not be able to enter their panels:
Do not forget, that accounts directory with all sub-folders and files should have been set with the full permissions (chmod 777), this applies also for settings.php.
The reason why this screen may appear is that you have exceeded the maximum number of installations. Generally, the license agreement only allows you to install Post Affiliate Pro 4 on at most 3 different domains/subdomains/IP addresses and use it only on one. It may happen, that you would like to test the application before the full run. Note, that if you exceed the maximum number of possible installations, you will have to purchase a new license (you could still have an option to use Post Affiliate Pro on one of the previously used 3 domains/subdomains/IPs).
Another reason might be your server, which blocks the communication between your application and our server that validates the license. Please, try to unblock the communication of your server with .postaffiliatepro.com domains.
It also happens, that your server settings might handle www. prefix as a subdomain and Post Affiliate Pro might run only on https://yoursite.com/affiliate and would cast an invalid license for the address https://www.yoursite.com/affiliate. Please, change the server settings in order www. prefix would not be handled as a subdomain.