CauseMost likely the IP address is assigned as a dedicated IP for the control panel. As you know dedicated IP can be used for only one domain and Parallels Plesk Panel (PP) is already using it. That is why you are not able to see that IP address in the "Change Hosting Settings" tab for the domain.
You have two options:
1. If you change the IP address type to Shared, it will become visible for the "domain.com" domain hosting setting and you can assign it from the "Change Hosting Settings" tab
2. If you want to make the IP address dedicated for the "domain.com" domain, you can change the main IP of the control panel. After that you can use the planned IP for the "domain.com" domain.