If you want to use Elementor to make custom templates, you must use Elementor - Theme builder and this requires Elementor PRO

You can't use Gutenberg custom templates inside Elementor. Gutenberg custom post templates are working only in Gutenberg editor

see screenshots attached:
1. screenshot (163): i want to create a post template (see the yellow mark up)
2. screenshot (164): this is the post template i created that i wanna use for every upcoming post
3. screenshot (165): now i created a new post and i set the self created post template as template for this post (see yellow mark up on the right)
4. screenshot (166): i want to edit this post now with elementor but this error message appears saying i have to implement the "the_content"-function.