The constraint specified in this statement cannot be found. Review the spelling of the name shown. If it is as you intended, check the names of existing named constraints by querying the sysconstraints table as shown in the following example. If the owner name shown for the desired constraint is not yours, you need to be a Database Administrator to reference it (and, if this database is ANSI compliant, you need to qualify the name with its owner-name).
SELECT constrname, owner FROM informix.sysconstraints