As you are designing a new table, you select a data type for each column. Before saving the table, the data types can be modified as you develop the table; but after saving the table, some data types cannot be converted to a different data type. For example, a column might initially have a “bit” data type, and later you may decide the column should be a “uniqueidentifier” data type. In this instance, the Visual Database Tools cannot convert from a bit to a GUID.
To correct this error
Delete the current column and add a new column with the required data type.