If you double click the error in the console you get more details about the error(s). Which you can add to your initial question, this will provide us with more information.
My guess would be that you try to change the “TrainingEvent” object, but the first retrieve action did not return a TrainingEvent object (change object cannot be null). A good practice is to add a split, and check if an object exists before changing it. However in this case you should first find out why it fails, and why (if that is the case) the retrieve did not return a result
If you add a debugging point to your flow you can see at which step it fails and what the variables are (more info on the debugger)
Edit: the error shows: “Change object ‘TrainingEvent’ should not be null”, this means the first retrieve action did not return a result. You need to check the logic: the association between Registration and TrainingEvent needs to be set.