You create a model-driven app and open it in the Dynamics 365 mobile app or the Power Apps app (now supports model-driven). But when you click new record, you get a form but you cannot enter in data correctly — maybe there are some missing fields or something else is wrong.

Dynamics 365 mobile uses Quick Create to create records. If you don’t have quick create enabled or a quick create form created for the entity, Dynamics 365 will generate one for you based on the main form.

To quote the official documentation:

Dynamics 365 mobile apps use quick create forms for creating new records. If an entity already has a quick create form configured for it, the mobile apps use that form. If an entity doesn’t have a configured quick create form, Power Apps generates a quick create form for creating records in the mobile apps based on the main form definition.

https://docs.microsoft.com/powerapps/maker/model-driven-apps/create-edit-quick-create-forms

So if you don’t get the expected results, enable quick create on the entity and create a quick create form that contains the desired fields.