This could happen if there are insufficient funds in your account balance to cover the amount of the Risk-Free bet. Risk-Free bets require you to place them using your own real money, and if the bet loses, you will receive the funds back.
Another reason could be that you have an existing bonus active on your account. Risk-Free bets cannot be used simultaneously with an active bonus. You can verify this in the 'Active Rewards' section of your account.
Always remember to review the Terms and Conditions of your offer!