The DataGroup System Validator displays a warning message stating that the default value for the Slab On Grade/Direction Angle property (Display Name is "Reinforcement Direction") is invalid when the field is left blank. This is a System Level Datagroup property, so we've set it as both hidden and un-editable in the DataGroup Catalog Editor.
The DataGroup validation error is a result of there being no default value for the Reinforcement Direction property for that catalog item, and since the Editable flag is turned off it cannot be changed by the user during placement. However, whether the property is Editable or Hidden is not relative to the issue, but rather that the property must have a value since it is a dimensional aspect of the component.
As long as the Reinforcement Direction property does have a default value in the DataGroup Catalog Editor you can keep it un-editable and/or hidden without any further issues.