You say that you have PHP 7.4 but the error message you are receiving means that you actually have PHP 5. Kindly note that the minimum requirement for our current software versions is PHP 7.2.
Please go to your hosting control panel and change the PHP version for your site to PHP 7.4.
I'd like to bring your attention to something very important. The PHP version reporting by the hosting control panel (most likely you're using cPanel) is the one used by the hosting control panel itself, not necessarily the one used for your site. A server can have a multitude of PHP version installed on it. The default PHP version is usually a pretty old version (e.g. PHP 5.6) to cater for older sites. The control panel itself is using one of the newer PHP versions (e.g. PHP 7.4) for reasons of performance and, well, because newer PHP versions allow its developers to implement more features more easily.
The hosting control panel software also provides a means of changing the PHP version which will be used by your site. Selecting a PHP version there makes a change in your site's .htaccess file, adding a line starting with AddHandler or SetHandler which tells the server which PHP version you want to use with your site. If you change the PHP version of your site and then make modifications to your .htaccess file which remove that line your site will revert to the default, older PHP version which might break your site.
Please note that updating Akeeba Backup DOES NOT and CAN NOT change your .htaccess file. Restoring a backup MAY do that (it's an option in the penultimate step of the restoration titled “Site Setup”).
Nicholas K. Dionysopoulos
Lead Developer and Director
🇬🇷Greek: native 🇬🇧English: excellent 🇫🇷French: basic • 🕐 My time zone is Europe / Athens
Please keep in mind my timezone and cultural differences when reading my replies. Thank you!