The Problem is:
It says that the domain name is already added. But I just bought the domain and changed the Nameserver to this but it says that the domain is already registered here.
The message: The selected custom domain is already in use on freedomainhosting.info. Please remove the custom domain from your account at freedomainhosting.info before you can host it on InfinityFree.
That message means that there is another reseller on the platform who used the domain freedomainhosting.info to sign up. That could be you or a previous owner of the domain name.
Regardless of who added the domain, you’ll need to contact iFastNet at https://support.ifastnet.com and ask them to remove the domain for you.