1. Right click SA Folder, choose "Properties" at the bottom and untick read only if it's selected.
2. That version of IMG Tool can't add two files to SA IMG at once, so to install it delete the original NRG500.TXD and DFF then add NRG500.txd and NRG500.dff separately. Never use the "replace" function, always delete the original files then add the new ones.
I mean it looks like you have two NRG's installed which would crash the game