Question about Microsoft 365 Family

Last year I bought the Single Version or extended it and also bought the Family Version (1 year licence). Last week I bought a laptop and a desktop for my kids and installed the Family version on it today. Now I wanted to enter the product key of Family in my Office account, but this is always rejected with the message "This product key has already been used". Yet we have only carried out two installations or registrations. Why can't I extend my existing Single Version by 1 year with the Family Version activated today? Or is there another problem? Who has an idea?