Here the reply from Elementor support.
Elementor removes the single option since the single option will automatically ask you for a page or post to choose from. Currently, you can create a single post and set the display condition to the Portfolios or any other CPT UI post type. I have tried to create a screencast by adding a single post builder and have assigned the display conditions only to show the post for “Portfolios”. Please refer to the video screencast.
Before publishing the post, you will ask for the display condition, So this is the part where you need to define either it’s for Portfolio or any other CPT posts types or any categories. Please try to add display conditions to the single/archive templates and let us know how it goes.
For you ref:
https://elementor.com/help/create-archive-template/
https://elementor.com/help/conditions/
I hope this helps you out.