Custom dorms not listed while enrolling into university

Did you set the lot assigned to the correct setting? There are two separate Student Housing for this purpose. It will be called Universe Housing. If you are not sure where this is, it is in the same location where you choose the type of lot you want it to be such as a resident, gym, library, or the other choices.

This link goes into more detail.
https://simscommunity.info/2019/11/15/the-sims-4-discover-university-lot-types-lot-traits/