You cannot use variables, like @tableName, in DDL. Besides, splinting the name into part and ignoring the schema can only result in bugs. You should just use the ”?” replacement in the SQL batch parameter and rely on the MSforeachtable
replacement:
EXEC sp_MSforeachtable '
if not exists (select * from sys.columns
where object_id = object_id(''?'')
and name=""CreatedOn'')
begin
ALTER TABLE ? ADD CreatedOn datetime NOT NULL DEFAULT getdate();
end';