Since this is a general error that does not tell you the specific problem, you will need to select the entity in the Object Browser and then use the Editor button to open the Entity Attributes Editor. That should give you a more specific error message. If it does not, take a screen shot of your Entity Attributes Editor and paste it here. It is likely that you have not completely defined all the attributes on your entity. For example, you may have a field that does not have a "FLD length NBR" triple because you may have forgotten to inherit from another field for one of your attributes.
Hope that helps!
As Jill mentioned, some of the fields may be missing "data type or length".
To generate table SQL script that information is mandatory.
David, did Jill's suggestion help? Let us know!