HI, i've a problem to complete the "Build a Conference Management App", pat Creating Apex triggers. All steps are completed, functionaltiy works as it should be, but receives this error.

Step not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Insert failed. First exception on row 0; first error: FIELD_CUSTOM_VALIDATION_EXCEPTION, The speaker is allready booked at that time: [] 
Note: you may run into errors if you've skipped previous steps.

What I'm doing wrong ?