Unable to approve market sale or trades on Steam Desktop Authenticator.

Or they could just google it. It was an incredibly easy answer to find- I just plugged in the last four words of the error and the app's name, and all of the first few results I got were explanations, some directly from the app's developer, along with recommendations for suitable methods of progression on the matter. Took all of 20 seconds to get the answer and options and understand them, and only about another minute to look up supporting information (as per the link below).

Basically, Authenticator doesn't work with Win 7 anymore due to Steam ceasing support for older versions of Windows in January of this year (ie, Steam is expecting security settings that don't exist on Windows 7, and SDA isn't configured to work around that), but according to posters on the topic there are similar services that do still have Win 7 compatibility, if an alternative is desired.

However, Steam's FAQ, linked above, suggests that Steam itself may potentially not work too much longer on older Windows, so ultimately the best fix would be to switch to Windows 10 or 11.