If you encounter this error, it is possible that the node was previously added to the cluster but not properly cleaned up. You can check the following three directories:  

Check if there are any leftover files in the directories `/etc/pve/qemu-server` or `/etc/pve/lxc` on the node you are trying to add:  

ls /etc/pve/qemu-server  

and  

ls /etc/pve/lxc  

as well as  

ls /etc/pve/nodes   

In the `/etc/pve/nodes/` directory, I found that each old node (e.g., `/etc/pve/nodes/node1/qemu-server/`) had a corresponding directory containing a VM configuration file, such as `101.conf`. After I deleted these files, the connection worked successfully.