Check to make sure that there is not an apostrophe or other special character in the Raisers Edge event name. If so, edit the event name and remove the apostrophe or special character and attempt to process the transaction again. 

If that is not the issue try this workaround: 

1.  Open the registration transaction
2.  Click on Find Event
3.  Click in the Event field and press F7 to find the event
4.  Select the event from the search tool
5.  Click OK to accept the event selection

You will likely encounter a prompt asking you to create a new event.

6.  Select No - as in you will not create a new event
7.  Click in the Event field again and press F7
8.  Locate the same event through the event search
9.  Click OK to submit the event selection a second time

You should find that the selection is accepted this time.

If neither option above applies:

Contact Support and reference this article. 

Include screenshots of the error message and the steps you are taking.