Assuming that you’re seeing the following message after clicking on Activate:
Error: Current PHP version does not meet minimum requirements for Easy Add Thumbnail.
That’s WordPress preventing you from activating the plugin for a site running a PHP version older than 7.0. The PHP version required is set in the plugin readme.txt, and WordPress takes this value to know the PHP version the plugin is requiring.
As said Easy Add Thumbnail doesn’t has any control over your site PHP checking or displaying the error message, that’s entirely handled by WordPress. So if you’re sure your site is running PHP 7.0 or newer and you’re still getting that message, that’s a WordPress issue not a plugin issue, and you should report this to the WordPress core team or ask for help in the WP troubleshooting forum.
That said, the plugin may work with old PHP versions, but I don’t support using them. So, if you want to still use the plugin with older PHP versions, you can delete the readme.txt file using FTP or a file manager. That should allow you to activate the plugin with any PHP version at your own risk.
If you’re not seeing the above error message but something different, you will want to investigate the issue further and ask for support to those security plugins you think are related. In any case this is not an Easy Add Thumbnail plugin issue.