oops error when I try to use the upgrade functions module

[geeuk]

Hello All,

I am new to whmcs and I am experiencing an issue when select the upgrade package of a product I get the following error message:

Oops!

Something went wrong and we couldn't process your request.

Please go back to the previous page and try again.

Error: [] operator not supported for strings in /var/www/html/includes/upgradefunctions.php:0
Stack trace:
#0 /var/www/html/upgrade.php(0): SumUpPackageUpgradeOrder('10', 2, 'monthly', NULL, 'paypal', true)
#1 {main}

I am using apache2 PHP Version 7.2.24-1+ubuntu18.04.1+deb.sury.org+1

Any suggestions please?

Thanks,

Gee