You were trying to place the order before the market opened.
On your order presets is a box that says, "Allow trades outside RTH."
Your box may not be checked.
As well, a long-standing glitch in TWS denies outside-RTH trades from transmission unless the entire Presets page is deleted and re-written.
It lasted 15 minutes, it would have been a winning trade, I would have finished my trading for today. Later I placed a less than ideal order, now I'm considering to cut my loss. I'm angry.