If you want to publish a package on npm using the name @npmtestscope/firstpackage, you need to make sure that the namespace @npmtestscope exists on npm. To create that namespace, you need to create an organization on npm with the name npmtestscope.
After you have created the organization, you can publish your package named @npmtestscope/firstpackage by executing npm publish --access public.
Note: To run npm publish for a package that belongs to a npm organization, you need to be logged in on your computer as a member of that organization. You can do that by executing npm login. The npm whoami command will show you the user name associated with the current login.