The Price Check window shows the contracts but none are selected?
If there are valid contracts shown in the Price Check window, but none were selected, it could be the “Use Customer Pricing even if Special Price is higher” option in the Omni Price Setup (Cards >> Omni Price >> Setup Omni Price) which is stopping the contract from being selected. This option must be checked if you want to force a Contract ID to be used when the contracted price is more expensive than the price from standard pricing.