Cannot edit line item unit price in Sales Order in B1

I have a sales order record with 1 product item (line 1) in B1,

For this sales order, I can add a new item line with normal operation (e.g. change quantity, unit price, delete row).

However, for line 1, I could only modify the quantity value, where I cannot do the update in unit price. Furthermore, even I select and right click line 1, I cannot find any "Delete Row" option in the menu.

When I check with the error code, which only shows:

[RDR1.Price][line: 1] , 'Field cannot be updated (ODBC -1029)'

I tried another Sales Order where I could successfully do the row deletion or update tasks.

Is there anyone could advise, what is the possible reason for I cannot do the item unit price updation?

Thank you very much.