Support

Please see this article https://www.akeeba.com/news/1744-advice-for-install-probs-with-march-2021-software-releases.html

We discontinued Akeeba Subscriptions as a commercial extension back in November 2013 and stopped offering any kind of end user support for it in June 2014. We told you to move to a different extension as soon as possible, see https://www.akeeba.com/news/1534-the-future-of-akeebasubs.html. That was 5 1/2 years before version 7.

We made it clear since Akeeba Subscriptions 5 (February 2016) that we consider it an internal project and we will be gradually removing any features we are not using. We stopped listing it on our site at the same time. That was a bit over 3 years before version 7.

We said that you must urgently move to a different subscriptions component since version 6 (February 2018). That was more than a year before version 7.

In April 2019 we completely discontinued Akeeba Subscriptions. Version 7 is an internal product, specific to our own site only. You can't download it and you can't use it. So, your problem is with a version of Akeeba Subscriptions that was clearly marked as INTERNAL PROJECT – NO SUPPORT released more than 2 ½ years ago (predating, therefore incompatible with Joomla 3.9 and PHP 7.4) and at least 4 ½ years after we said we no longer support Akeeba Subscriptions and you need to urgently move to something different.

Moreover, FOF 3 itself is discontinued since March 2nd, 2021.

I'm sorry. I cannot help you with software which was NEVER supported on your Joomla and PHP versions and you were explicitly told not to use and not to expect support for. You should have migrated to a different subscriptions component 6 ½ years ago.

Restore your site from a backup, uninstall Akeeba Subscriptions and proceed with all the updates. If you don't have already a different subscriptions extension kindly note that you had 6 ½ years. This is an eternity in computer terms. Even a year's notice is considered incredibly generous. 6 months is plenty. 3 months is standard.