but it keeps saying, "You can't use the shield when your march is doing war related operations".

I'm extremely new to this game and it's my brother's app game. He's at boot camp so he left it up to me to activate peace shield. Well with one week away till his graduation and with a string of bad luck I ended up forgetting to active it and now I'm worried that he will lose all of his effort.
I placed a defense boost 12hr though. So I don't know if that would help.