I'm using TeamViewer 15 but unable to remote login my client PC running with version 9...
Hello @Michael_Pang,
Great to see you posting for the first time! 🚀
Indeed, it is not possible to connect to version 9 anymore, as it has been phased out. Please see our announcement here: Complimentary TeamViewer 15 access for legacy versions to be deprecated
The oldest version you can connect to is version 11. However, it requires a professional license. Using TeamViewer with the free version is only possible with the latest TeamViewer (15 so far) installed on both devices.
Let me know if you have any further questions! I'll be happy to assist.
/JeanK