@CROSS Com
Probably means you have not created a VPC network in your DO project.
As I remember you simply have to go into your DO dashboard > Networking > VPC > Select your region and then create one. I think there's an option to have it automatically create an IP range for you so that you avoid having to configure everything manually.
Once done you should be able to select this network while deploying through 3CX to Digital Ocean. If you have trouble with it it might be wise consulting DO's documentation on creating VPC networks.