npx or similar commands not working as expected?

Any tips on how to handle the below issues?

```
$> asdf --version
v0.7.4
$> asdf which npx
/Users/joshua/.asdf/installs/nodejs/11.10.1/.npm/bin/npx
$> npx

ERROR: You must supply a command.

Execute binaries from npm packages.

npx [options] [@version] [command-arg]...

npx [options] [-p|--package ]... [command-arg]...

npx [options] -c ''

npx --shell-auto-fallback [shell]

Options:
--package, -p Package to be installed. [string]
--cache Location of the npm cache. [string]
--always-spawn Always spawn a child process to execute the command.
[boolean]
--no-install Skip installation if a package is missing. [boolean]
--userconfig Path to user npmrc. [string]
--call, -c Execute string as if inside `npm run-script`. [string]
--shell, -s Shell to execute the command with, if any.
[string] [default: false]
--shell-auto-fallback Generate shell code to use npx as the "command not
found" fallback.
[string] [choices: "", "bash", "fish", "zsh"]
--ignore-existing Ignores existing binaries in $PATH, or in the local
project. This forces npx to do a temporary install and
use the latest version. [boolean]
--quiet, -q Suppress output from npx itself. Subcommands will not
be affected. [boolean]
--npm npm binary to use for internal operations.
[string] [default:
"/Users/joshua/.asdf/installs/nodejs/11.10.1/.npm/lib/node_modules/npx/node_mo
dules/npm/bin/npm-cli.js"]
--node-arg, -n Extra node argument when calling a node binary.[string]
--version, -v Show version number [boolean]
--help, -h Show help [boolean]

For the full documentation, see the manual page for npx(1).

$> npx create-react-app
asdf: No version set for command create-react-app
you might want to add one of the following in your .tool-versions file:

nodejs 8.3.0
nodejs 11.6.0
$> which npx
/Users/joshua/.asdf/shims/npx
```

Use the reported `asdf --version`, `asdf which npx`, `npx`, and `npx create-react-app` commands as reproduction steps, comparing their results with `which npx`. Done means the commands resolve consistently under the selected Node.js version, with the required configuration or limitation clearly documented.

Written by the indexing model from the issue text.