PowerShell: The term 'x' is not recognized as the name of a cmdlet, function, script file, or operable program

The term 'Get-Foo' is not recognized as the name of a cmdlet, function, script file, or operable program. means PowerShell found nothing with that name: no alias, function, cmdlet from an available module, script or executable in PATH. Check the spelling with Get-Command, then install or import the module that provides the cmdlet, or add the program to PATH. A script in the current folder must be called as .\script.ps1.

Reproduce it

'"hello from script"' | Set-Content hello.ps1
foreach ($cmd in 'Get-Foo123', 'hello.ps1', '.\hello.ps1') {
  try { & $cmd } catch { "$cmd -> $($_.Exception.Message)" }
}

Get-Foo123 -> The term 'Get-Foo123' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
hello.ps1 -> The term 'hello.ps1' is not recognized as the name of a cmdlet, function, script file, or operable program. Check the spelling of the name, or if a path was included, verify that the path is correct and try again.
hello from script

hello.ps1 fails and .\hello.ps1 runs: PowerShell does not search the current folder, unlike cmd. In an interactive console it also prints a suggestion to use .\hello.ps1.

Find the command

Get-Command Get-ADUser -All          # is it anywhere on this machine?
Get-Command *mailbox* -Module ExchangeOnlineManagement
Get-Module -ListAvailable ActiveDirectory
$env:PATH -split ';'

Call a program with spaces in its path

A quoted path is a string, not a command. Use the call operator:

& 'C:\Program Files\Tool\tool.exe' --version

Without &, PowerShell prints the string instead of running the program.