The PayStack payment gateway is 100% incompatible with FOSSBilling version 0.5.0 and newer regardless of what theme you are using. The only reason it is functional for @Oluwatobi is because of a leftover
Box\InjectionAwareInterface class on the disk from an older version of FOSSBilling / BoxBilling.
It can be used on a newer version of FOSSBilling by manually creating
InjectionAwareInterface.php under the
/library/Box/ directory and placing the following contents in it:
* @param \Box_Di $di
* @return void
public function setDi($di);
* @return \Box_Di
public function getDi ();
This will work indefinitely, but the only proper fix is for the author of the payment gateway to update maybe 6 lines of code for it to genuinely be compatible.