Just as the message says, you already are using that domain on another account. Make sure you have removed the domain from the other account. Note that it also may be in use with another host, so not necessarily InfinityFree.
I bought the domain name from another provider and registered it in infinityFree though after registering for hosting, the client area of my account is inaccessible since it displays the screenshot below.
no at first before creating the account on infinity free it gave me an option to add it and it asked to wait for 24hrs then …after 24 hrs no account was created when i logged in and it displayed that the domain name is already in use on infinity free
while i cannot the access the account since there was no account created. There is no hosting plant or services in the domain provider.