Unable to Untick Read Only child table field

I have my Quotation Form, and i add a item to Quotation Item, and im not able to change the field “Costo Unitario” (price_list_rate).
I Have some products without price, so i have to add that product and set a price in that “costo unitario” field. But im not able to change the value, the field is disabled.

If I open the Edit Grid, Im able to edit the field, but not in the small table view.

So… i went to my Quotation Item DocType, and i wanted to UNTICK the “READ ONLY” field.

and i couldnt, a error message comes…

Im Logged in as Administrator.

hello, you can try Customize Form

Yes, I already customize my doctype and tried to uncheck the “read only” field. but i couldnt, as you can see in the picture i uploaded.
my version is 14, i had the version13 before and i had no problem with that version, the problem cames when i update to 14