I am getting error message as “Sorry, there aren’t so many seats available!”

I enabled Event Manager Plugin after that I added the code <p>#_ADDBOOKINGFORM</p>
For Event Reservation form for Registering particular Events.
After giving the details and submit in the form I am getting this error
“Sorry, there aren’t so many seats available!” I observed in the code in this event_id is not coming.
Please help me to how to fix.
Thank you