(Solved) CiviEvent mandatory key missing error
As Jon said, its difficult to say. But its certainly because of com.pesc.sparkpost extension. I believe the extensions is creating activity of type 'Transactional Email' which either missing of disabled. 

Troubleshoot:
Check under Administer >> Customize Data and Screens >> Activity types if you can see 'Transactional Email'. 

If you can't then add new Activity type named 'Transactional Email'

If you can see then 


Check if its enabled.
Check the machine name for that activity type
either in database or using api explorer using params shown in screenshot (civicrm/api). If you dont get result then create a new one.




Thanks
Pradeep