There are a few things you need to remember to solve this issue Let's check step by step :
1. Is virtualizor installed on your slave server.
2. Make sure the IP address you entered is correct. double-check it.
3. make sure you entered the server API password correctly. If you don't know the server API password. please go to virtualizor admin panel then > configuration > server info. grab the API password.
4. try curl -kv from both server if you face any issue then maybe its firewall issue or network configuration issue.
If you tried above steps and you'll still face issue then do let us know. Thank you