Check whether you truly have not left the domain staying assigned to one of your accouns on InfinityFree and make sure that it is fully detached from the previous hosting on which you used the domain (the domain being unregistered and unassociated from that account).
Next time, could you please not throw away the topic template, but fill it in instead? That way, you don’t forget to include important information. Like which domain you’re actually trying to add, which is a pretty important detail when trying to figure out why a particular domain is being reported as in use.