Thanks for your detailed report.
I've just re-packaged 6.5.7 as 6.5.8 without the 'sr' language.
Issue 2460 – Cannot install from Microsoft Store because of invalid 'sr' language

Thanks for the heads-up; yes, I was aware. I tried installing the appx and ran into the same error the chat mentioned to you!

Application installation failed with error message: MartinPikryl.WinSCP_6.5.7.0_x86__tvv458r3h9r5m : error 0x80070057 : cannot register package MartinPikryl.WinSCP_6.5.7.0_x86__tvv458r3h9r5m because the specified resource language 'sr' is invalid. (0x80070057)

This is a packaging bug in the Microsoft Store build of WinSCP 6.5.7, not a problem with your PC.
What it means
0x80070057 here is Windows rejecting the Store package because its AppX manifest lists resource language sr (Serbian). The Store language list includes sr, but package registration does not accept the bare tag. It wants a full tag such as sr-Latn-RS or sr-Cyrl-RS. Same failure has hit Bitwarden, Rufus, and others.
WinSCP 6.5.7 (released today) completed the Serbian translation. That is almost certainly when sr was added to the Store package.
What you can do now

Use the classic installer (best workaround)
Download 6.5.7 from winscp.net and install that. It is not an AppX package, so this language check never runs. Your settings can stay; Store and classic installs can coexist, but pick one as the daily app.
Wait for a Store republish
Martin Prikryl has to change the Store manifest from sr to a valid BCP-47 tag and push a new package. Until then, every Store install/update of 6.5.7 will fail the same way.
Do not waste time on generic 0x80070057 fixes
wsreset, Store reset, SFC, language pack installs, etc. will not help. The package itself is invalid.

If you need Serbian in the UI after using the classic installer, WinSCP’s own translation is the srl plugin on their translations page, not the Store sr tag.

During some research into the failure with ChatGPT eventually I do see an error:

0x80070057
Cannot register ... because the specified resource language 'sr'
is not a valid language.

Hi Martin,

I'm experiencing an issue with the Microsoft Store version of WinSCP.

I initially tried to update WinSCP several times through the Microsoft Store, but the update repeatedly failed. The Microsoft Store displayed the following message:

Something happened. We're aware of the problem and we're working to fix it. Try again later.

Error code:
0x80073CF6

The package identified in the error is:

After restarting Windows, the update was eventually installed successfully. However, after the update, WinSCP stopped launching.

I tried launching WinSCP from the Start menu and also by clicking Open directly in the Microsoft Store, but nothing happened.

I then tried the Repair/Reset option in Windows Settings, but Windows reported that the application could not be repaired/restored.

I subsequently uninstalled WinSCP completely and tried to perform a clean installation.

However, the clean installation from the Microsoft Store also fails with the same error 0x80073CF6.

The complete sequence was:

• WinSCP repeatedly failed to update through Microsoft Store with error 0x80073CF6.
  
• I restarted Windows.
  
• The update was eventually installed.
  
• After the update, WinSCP stopped launching.
  
• The Repair/Reset option reported that the application could not be repaired/restored.
  
• I uninstalled WinSCP.
  
• I tried to reinstall it from the Microsoft Store.
  
• The installation failed again with the same 0x80073CF6 error.
  

At this point, I cannot install the Microsoft Store version of WinSCP at all.

It seems there may be an issue with the deployment or installation of the MSIX/AppX package distributed through the Microsoft Store.

Could you please let me know if this is a known issue with the Microsoft Store package, or if there is a recommended way to diagnose what is causing the 0x80073CF6 error?

I can provide Windows version/build information, logs, screenshots, or any other information that may help investigate the issue.

Thank you!