Hi, @nyacumo please your PHP configuration to generate the query before you edit it.
SELECT CONCAT(i.Nombre,' ',i.Apellido) AS label, i.id AS value FROM client AS i
The issue was with the value and label, if you were to select the field and the tables using the Datasource Look Up or Data Table before you edit the query you should have seen the structure phprad uses to get the value and label. The above query should work but I don't know if the field id exists in your client database table. Please use the right field there, if I am wrong.