There are some reasons of why you are experiencing that issue:

1. Your backpack might be outdated, try closing TF2 if you have it open or try restarting Steam.
2. The item you are trying to sell is not on the price list. Scroll down on the item banking page to see it.
3. If the item is uncraftable, make sure to check the chart to see if we accept the not craftable version of it.
4. The item is untradeable.
5. We have hit the stock limit of said item. You can see the limits in the price list.