Looks like you are trying to change Field Type for a Standard field, which is not allowed by the system.
You can create a Custom Field as a work around
how can i create a custom field?
Ok, i got it.
Another question, how do i create a “select” type which can have month and year?
You can use the “Date” field instead. However, it will be shown in dd-mm-yyyy (or whatever format you have set in System Settings).
Hope this helps.
Thanks, but i can’t use date and system setting.