Why am I unable to modify order price lines in TWS charts?

You can set up stop orders on individual positions on your account, but not a daily stop loss based on the cumulative daily loss of your entire portfolio value.

Information on how to use stop orders…

You can Attach a Stop Loss order when you create an opening combo order in TWS.

In the Mosaic Order Entry window, click on the Advanced button and select Attach Stop Loss/Profit Tacker/Bracket. Place a check next to Stop Loss, enter the order parameters and submit…

To create presets for your orders, in both Mosaic and Classic TWS you will want to access Global Configuration and select Presets. 

You can configure profit-taker and stop-loss order presets in the areas labeled "Target Order (Profit-taker)" and "Attached Stop Order", respectively…

Depending on your configured Order Presets you may not be able to modify the price of your open Bracket order's attached legs. In the Target Order (Profit Taker) and Stop Loss Order sections of the Order Presets window there are check boxes for "Apply Offset to Parent". If you check this box the Limit price for Attached Target orders and the Stop price for Attached Stop Loss orders will be set relative to the actual trade price of the Parent order. If your Bracket orders are configured this way you will not be able to edit the prices for the child Stop Loss or Profit Taking legs of your active Bracket order. If you want to be able to modify the prices for the individual attached child legs of a working Bracket order, then you need to leave the boxes unchecked.  …