There is an existing A record for the relevant hostname from where you are trying to add the forwarding. This needs to be removed first before you can add the forwarding. Please follow the below steps.
1. Go to your domain dashboard, find the domain name.
2. On the right side click "Manage".
3. Under DNS Manager click "Edit".
4. Under "DNS Record", find the A record/s related to the domain name you are trying to forward and delete the A records.
5. You may try the domain forwarding again.