Creating a GitHub Action which runs a Powershell script on a Windows host

The error message implies that you cannot refer to your script with a relative path:

The command is passed to pwsh, PowerShell [Core]'s CLI, via its -Command (-c) parameter, which interprets the install-tf-win part of
install-tf-win\install-tf-win.ps1 as a module name rather than as a subdirectory in the absence of an actual subdirectory by that name on Windows^[1], so the implication is that such a path doesn't exist.

The linked documentation, suggests that you need an absolute path, based on the GITHUB_ACTION_PATH environment variable, which in PowerShell must be referenced as $env:GITHUB_ACTION_PATH (untested):

# ...
    - run: '& $env:GITHUB_ACTION_PATH/install-tf-win/install-tf-win.ps1'
      shell: pwsh

Note:

• The need to use &, the call operator, which is a syntactic necessity due to the script path containing a(n environment-)variable reference; the same would apply if the path were quoted - see this answer for background information.

• Since & is a metacharacter in YAML when used unquoted at the start of a value, the entire string is quoted. Single-quoting ('...') is employed in this case, so that YAML doesn't interpret the contents of the string up front.

  • As an aside: The implication of '...'-quoting working (as confirmed by Kevin, the OP) is that when pwsh -c is ultimately called on Windows, the string's content is (properly) double-quoted ("..."), because PowerShell's CLI only recognizes " as having syntactic function for command-line argument parsing. By contrast, a '...'-quoted -c argument would be interpreted as a verbatim string rather than as a command, causing its content to be simply echoed.

[1] How PowerShell interprets a path such as foo\bar.ps1 when executed as a command, as of PowerShell 7.1

Interpreted as a command - both inside a PowerShell session and via the -Command / -c parameter of the PowerShell CLI, as used by GitHub Actions - the form foo\bar.ps1 is ambiguous:

• It could refer to a module named foo, and its bar.ps1 command (even though module commands don't have .ps1 name extensions).

• It could refer to a subdirectory foo and file bar.ps1 in it.

  • This applies to Windows only:

    • PowerShell first interprets the path as a file-system path.
    • If the path doesn't exist, it falls back to interpreting it as a module-command reference.

  • On Unix-like platforms - even though PowerShell generally allows you to use \ and / interchangeably as the path separator on all supported platforms - this particular path form is always interpreted as a module reference; in other words: you cannot use this syntax form to invoke a script this way (see below).

  • This surprising inconsistency is discussed in GitHub issue #14307.

There are two ways to avoid this ambiguity, which work on all supported platforms; the following cause PowerShell to unequivocally treat the path as file-system script path:

• Use / instead of \: foo/bar.ps1

• Prefix the path with .\ (or ./): .\foo\bar.ps1

(Of course, you could also use a full path.)

Note that when the -File parameter of PowerShell's is used (rather than -Command / -c), this ambiguity doesn't arise, as the argument is then always considered a (potentially relative) file-system path, even on Unix-like platforms, irrespective of whether you use \ or /.