Unable To Select New Created Doctype In Customize Form

We Are Not Getting Any Option To Specify Doc Type As Master. Also We Are Unable To Select New Created Doctype While Customizing Form. Please Advice On This.

Which option you’re not getting?
You’re unable to select custom doctype in Customize form or you don’t see new changes you’ve made in Customize form?
Could you please elaborate on the issues you’re having?

If you have created a Custom Doctype, edit it from the Doctype list. Customize form is a view to edit only the standard doctypes.