This is because the Visual Database Tools validates the existing data with the constraint when you attempt to add it. But no problem, you can to decide do not to validate the existing data by changing the property Check Existing data On Creating or re-enabling to NO, but these data will be inconsistent with new rule. Or by using the NO CHECK clausule on ADD CONSTRAINT statement.

A recomendation is to review the data and change it to accept the changes and the new rule.

Regards,

([Permission]='operator' OR [Permission]='Supervisor')

for anyone who may need it.