Event Registration on Behalf of a Contact is Broken
This error usually occurs when the option values for Custom data type are disabled or deleted. Make sure you have all the options values required for event are enabled for Custom data type option group. Here are the options


If a option value is missing then you will need to add the option value either through UI or api. However for some option values its not possible to add same options with same machine name so it is always better to add such options through API Explorer. 

Navigate to CiviCRM >> Support >> Developer >> API Explore. And use the options shown in the image below once you are done with selection then click on execute button for api to process the params you selected.


HTH

Pradeep