I’ve followed the steps of https://ghost.org/tutorials/create-a-custom-post-template
But I can’t select the template in the post settings sidebar, because the template dropdown is not visible!
I’m using the latest version 5.54.
How are you making changes to your theme? If you’re editing/uploading files directly on the server did you restart Ghost after adding the custom template file?
And you made sure that the filename starts with custom- right, and ends in .hbs? 
I debugged a problem for someone last week that had an extra space in the filename after the .hbs. And so it wasn’t a .hbs file any more, and Ghost didn’t recognize it!
I’m guessing Kevin’s right (well, of course) and that you just need a restart, but if that’s not it, I’d triple check that file name.
I did: ghost restart - but always the same. I had another theme with “create-ghost-theme”, because I need tailwindcss.
But when I switch to Casper - always the same.
Now I installed ghost from scratch → and that worked, the template-drowdown is visible :-)
I was only testing (ghost install local) the first time, because I’m looking for a cms that’s suitable for me → for landing pages and smaller websites
I made websites with typo3, drupal, vue/nuxt with headless cms (strapi, directus) and tested a lot (apostrophe … https://jamstack.org)
ghost looks fine :-)