I am using the Strapi v3.0.0-beta.18.7 when I am trying to open the /admin pages I got 404 erorr page.
To run strapi I am using command:
../node_modules/.bin/strapi develop --watch-admin
Update your version then use strapi develop
in the beginning.
There will be text on the console before Project Information table
Check this text it will help.
Admin development at http://localhost:8000/admin/