As you have an unmanaged class account, restrictions come from two places.
Some restrictions are applied by the Machine Learning for Kids website (in other words, me).
For example, number of students, number of projects, etc.
One restriction is applied by the API key that you provide.
Because you provided a free API key for Watson Assistant, your agreement with IBM Cloud/Watson is that they will let you create five machine learning models at a time. Once your students collectively hit that limit, they won't let you create additional models until you delete one.
Hope that helps make what happened clearer.
Kind regards
D