On Transact SQL language the Msg 173 Level 15 - The definition for column must include a data type - means that you must write the data type for the new column.
Msg 173 Level 15 Example:
Invalid alter table:
USE model;
GO
ALTER TABLE students ADD dep_id;
GO
| Message |
|---|
| Msg 173, Level 15, State 1, Line 1 |
| The definition for column 'dep_id' must include a data type. |
Correct alter table:
USE model;
GO
ALTER TABLE students ADD dep_id INT;
GO
| Message |
|---|
| Command(s) completed successfully. |