Custom Domain

Acording to aternos custom ip i tried freenom but it dose not work freenom always shows an error when i try it please help

Either no domain like kfhdidbekdbkdbdkdbd.tk or anything else is not avilable or if its avilable then it show technical issues erroe on checkout after chosing neservers if i am logged in domains avalible when i am not logged are are unavilible when i am logged in