Hi Luana,
If you set the Editable property of the “No.” field, then you’re also not able to edit the number field when you create a new record. So don’t do that, unless you’re only assigning new customer numbers by using the number series.
Otherwise you should consider adding this code to the on validate field of the customer no.:
IF ("No." <> xRec."No.") AND (xRec."No." <> '') THEN
ERROR('You cannot edit the %1',FIELDCAPTION("No."));
Remember to set a similar code on the onRENAME trigger.