Hi everyone, in my second-hand 2014 V60, the manual shift mode can only upshift but not downshift. Is there a problem with the transmission? Any advice?

For safety and to protect the transmission, even in manual mode, the transmission doesn't unconditionally follow manual shift up or down commands. It makes decisions based on engine load, RPM, vehicle speed, and other factors. Manual shifting is just a request signal, not a command signal. The transmission should be fine.