Here, on click of “NEW” button of lookup view you are getting this message because on click of “NEW” button you are redirected to the “Main” form of related entity instead of “Quick Create” form. So, here you need to check below things:
1. “Allow Quick Create” option must be checked for related entity as shown below:

2. “Quick create” form must be present on related entity.
3.Related entity must be added in your Dynamics CRM App. If related entity is not added in your app then, on click of “NEW” you will be redirected to “Main” form of related entity by default instead of Quick Create form.
Please follow below steps, to add entity in Your Dynamics CRM App(i.e. Sale Hub).
Here, I have configured N:N relationship between “Account” and “Employee (Custom Entity)”. After that, I have added Subgrid of Employee entity on Account entity form. So, here I need to add this entity in Dynamics CRM App (i.e. Sales Hub).
Steps to add Entities and their components in Dynamics CRM App:
Navigate to Settings à My Apps and then Open your App in App Designer as shown below:

Then, go to Entities from Components and click on add entities icon as shown below.

After that, select related entity from entities list. Here, I have selected “Employee” entity as shown below:

Once you select your entity from entities list, the entity will be available in “Entity View” of your App as shown below.

Note: You can select specific components of entity by clicking on them as shown below. But, here to redirect to the “Quick Create” form of the entity on click of “New” button of Subgrid, you must select “Quick Create” form of that entity.
After that on click of “New” button of lookup view, you will be redirected to “Quick Create” form of related entity instead of “Main” form and will not get this message.
Hope this helps.
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Thanks!