I was able to confirm that this is indeed caused by file path length limitations:

1. In a Powershell prompt, run the following "cmdlet" to check whether long paths are supported or not (0: long paths not supported; 1: long paths supported):

Get-ItemProperty -Path "HKLM:System\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled"

2. Run the pip install command above (it should fail if LongPathsEnabled is 0)
3. Open a Powershell prompt with "Run as Administrator" to enable support for long paths:

Set-ItemProperty -Path "HKLM:System\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled" -Value 1

4. Run the pip install command above, which should now pass

For more information: https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation?tabs=registry#enable-long-paths-in-windows-10-version-1607-and-later

From more testing, I think I figured out the exact path that causes the error, at 262 characters when the default length without LongPathsEnabled is 260:

C:\Users\lbian\AppData\Local\Temp\pip-install-is8sb24x\idaes-pse_f27d74dd25594348bdef5015371f125f\build\bdist.win-amd64\wheel\.\idaes\core\surrogate\tests\data\onnx_models\net_st_net_5000_STM_100_s_2000000_60_5_tanh_1e-06_4096_tr_15481_Calcite_ST_idaes_info.json

Assuming this is true, and that the pattern for name mangling that pip uses is fixed in length, then the filename just happens to be long enough to go over the threshold by 2 characters 🙃

When I tried the above solution, i.e.,
Set-ItemProperty -Path "HKLM:System\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled" -Value 1
on a Windows virtual machine via Citrix, I get this error:

Set-ItemProperty : Requested registry access is not allowed.
At line:1 char:1
+ Set-ItemProperty -Path "HKLM:System\CurrentControlSet\Control\FileSys ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : PermissionDenied: (HKEY_LOCAL_MACH...trol\FileSystem:String) [Set-ItemProperty], Securit
   yException
    + FullyQualifiedErrorId : System.Security.SecurityException,Microsoft.PowerShell.Commands.SetItemPropertyCommand

When I tried the above solution, i.e., Set-ItemProperty -Path "HKLM:System\CurrentControlSet\Control\FileSystem" -Name "LongPathsEnabled" -Value 1 on a Windows virtual machine via Citrix, I get this error:

Set-ItemProperty : Requested registry access is not allowed.
At line:1 char:1
+ Set-ItemProperty -Path "HKLM:System\CurrentControlSet\Control\FileSys ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : PermissionDenied: (HKEY_LOCAL_MACH...trol\FileSystem:String) [Set-ItemProperty], Securit
   yException
    + FullyQualifiedErrorId : System.Security.SecurityException,Microsoft.PowerShell.Commands.SetItemPropertyCommand

Thanks for the feedback @fahim831. As I mentioned in my comment above, the Set-ItemProperty command needs to be run as administrator (which, depending on how your machine is configured, might not be possible to ordinary users).

Also, #1545 should have resolved this error, so commands such as pip install "idaes-pse @ git+https://github.com/IDAES/idaes-pse@main should work as expected now.

@lbianchi-lbl Thank you! I did run it with Admin privileges, which is why I was confused it didn't work. But the #1545 solution worked, using pip install git+ for both idaes and then watertap.