Solution
This error occurs due to the property you are going to define is restricted in Strict Type Definition. The only properties available for a Strict Type Definition control are those that affect the appearance of the control such as Visible, Disabled, Key Focus, Blinking, Position, and Bounds.
To fix this error, you may change your custom control (.ctl file) from a Strict Type Definitions (Strict Type Def.) to Type Definition (Type Def) and save it.