Welcome to the Microsoft Q&A Platform. Thank you for reaching out & I hope you are doing well.
I understand that you are facing issues while adding a VM to the backend Pool of a Load Balancer.
Based on the screenshot,
I am assuming that the Load Balancer is of basic SKU.
And there is an existing VM already associated with this Load Balancer and the new VM you are trying to add is not in the same availability set as the existing one.
This is an expected behavior with Basic SKU load balancer.
Refer : SKU Comparison
To mitigate this,
- You can either use a Standard SKU Load Balancer
- or Create VMs in the same availability set as the existing one
Kindly let us know if the above helps or you need further assistance on this issue.
Please don’t forget to close the thread by clicking "Accept the answer" wherever the information provided helps you, as this can be beneficial to other community members.