Then you execute the installer, you should have got a UAC prompt to allow running the installer as administrator:
https://winscp.net/eng/docs/ui_installer#uac
Haven't you? Is it a private computer? Or corporate?

Thank you for taking the time and trouble to reply. I am still trying to find my way around Windows 11 and while trying to do this I downloaded the setup.exe and found I could 'Run as administrator' – which as far as I can tell I already am - and lo and behold it worked.

Once more thank-you for responding.

Aren't there more details in the error message?
Can you post installer log file? Use /LOG command-line switch:
https://winscp.net/eng/docs/installation#automating

Hi,
I am pretty sure that this is a Windows 11 issue, but can't find a solution so wondered if anyone else has had the problem. I have only recently updated to Windows 11 and everything used to work just fine on Windows 10.
I am trying to update from WinSCP 5.19.6 to 5.21.2 but I get an unable to create directory message from the setup.exe

Setup unable to create the directory
C:\Users\bjane\AppData\Local\Temp\is-N41JF.tmp

I am an administrator and I have checked the permissions on the TEMP folder and everything looks good.