We recommend using RealVNC Viewer - it's free to use and guaranteed to work with RealVNC Server. You can download it here.
If you prefer to use another VNC Viewer for any reason, then you can use it to connect to RealVNC Server by making some configuration changes.
To connect from a non-RealVNC VNC Viewer to RealVNC Server, you will need to change RealVNC Server's Encryption from AlwaysOn to PreferOn and then reset your RealVNC Server's VNC Password.
There are two ways to do this:
Using the GUI. Open VNC Server's Options, and change the Encryption dropdown to Prefer On or Prefer Off, and click Apply.
Next, go to Users & Permissions and click the Password button on the right. Enter your password and make sure to check the "Allow connections from legacy VNC Viewers" checkbox.
Using SSH/command line. Run the below commands:
echo 'Encryption=PreferOn' | sudo tee -a /root/.vnc/config.d/vncserver-x11 sudo vncpasswd -legacy -service sudo systemctl restart vncserver-x11-serviced