The ALTER TABLE statement conflicted with the CHECK constraint
The error message is self-explanatory: Your check constraint is trying to enforce that all values in Datecom are at least one day earlier than Datelivr, but you must have at least one row where this is not true - either Datecom is the same day, later, or one of the values is NULL. Find those rows using:

SELECT Numcom, Datecom, Datelivr
FROM dbo.Commade
WHERE datediff(day,Datecom,Datelivr) &lt;= 0
  OR Datecom IS NULL
  OR Datelivr IS NULL;