This will fail for the value behind DEFAULT. It should be 'P'. You can add this in the editor, but why not add them automatically for string values?
CREATE TABLE [Clients] (
[id] INTEGER IDENTITY(0,1) NOT NULL,
[BeconID] INTEGER NOT NULL,
[Type] CHAR(1) NULL DEFAULT P,
[Lastname] VARCHAR(100) NULL,
This is by design.
You can put any expression in default field, it could be constants, so you must put literal values, Data Modeler won't parse the field content just to check which parts need quotes or not.