The server encountered an internal error or misconfiguration

Whenever I'm trying to create erp cash sale, I get the error listed below. Furthermore, whenever I tried to create erp item, the amount I enter in RRP Price and Selling Price does not get saved, when I click SAVE. Please help.

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator, support@supportwebsite.com and inform them of the time the error occurred, and anything you might have done that may have caused the error.

More information about this error may be available in the server error log.