Hi Partner,
Hope you are doing good.
I am sure you must have tried following these step, it would be great if you can verify the same
I just tried for Lead entity in Customer service Hub App and it works well, Steps I performed are mentioned here for reference:
1. Navigate to Customization Customize the System and look for Lead entity and check if "Allow quick create" is checked

2. Check in the forms if the "Quick create" form Exist in the Entity you are trying to add

3. Add this entity in Model driven App which you are trying to access, for example I am verifying in Customer service Hub APP
to Add it in model driven App -> Navigate to Customization -> Customize the system -> Model driven App and select the required APP
Once the App designer is open, Click on Entities and Select the required entity and from the Ellipses (i.e. Three dots) next to entity select all required assets

4 Once you select the entity, you should save the changes and Publish the same

5. Once the changes are saved and Publish, verify in app designer if you are able to see the lead entity in the list on left side and Quick create form is checked as in screenshot

6. Once you have this in place just close the App designer, Refresh the browser or logout and Login
7. Post refresh login, you should be able to see the Lead in quick create and this will open the quick create form
