From Frappe version 6.26 onward, users will not be allowed to re-arrange fields in a form unless they are custom fields. Here are the reasons:

1. ERPNext is a constantly evolving product and new fields will be added and arrangements will change, frequently. If the user has already re-arranged fields, this creates a conflict. New fields may be added out of context in such a case. All fields have labels that have a meaning based on the section they are based in, and if you move the field out of the section, the context changes and this could be disastrous.
   
2. The original motivation to allow rearrangement of fields was because there was no Print Format Builder. Now it is very easy to rearrange fields for printing.
3. Custom fields can still be placed where ever you want.

We hate taking back features, but in this case, this feature usually leads to a bad user experience and the original motivation also does not exist. This also restricts us from releasing new features and is a lose-lose all the way.

Example

Field arrangement messed up because of user re-arrangement