Craft 2 -> 3 Update /admin 404 I'm attempting to update a craft 2 site to craft 3.

Following these instructions, I used composer to install Craft:
https://docs.craftcms.com/v3/upgrade.html#if-you-want-your-directory-structure-to-resemble-a-new-craft-3-project%E2%80%A6 

I have got to the stage where I go to the domain.com/admin but I am getting the error: 


  The requested URL /admin was not found on this server.


Thanks for any help - at a dead end I am sure its something v simple!
Is a lot of potential reasons for what issue may be. Is Craft handling the 404 error? If not, it could be your server is not pointing to the web/index.php properly.

Can check that just putting a die('hello world'); at the top of your index file at least.