I am participating in a conference in China, but I cannot connect to VPN.

There is a possibility that VPN is restricted on your network, or that the VPN regulation by the Chinese government has been strengthened and connection is not possible.
https://lb-hikaku.com/post-709/

You may be able to connect using the SIM roaming service of a telecommunications company in Japan. Some on-campus systems can be accessed without using a VPN.