OnPremise Azure Devops Pipeline connecting to Artifact Feed via NuGet not working

I've tried this on two different OnPremise Azure Devops Servers and get the following error on both:

[error]The nuget command failed with exit code(1) and error(NU1301: Unable to load the service index for source https://.../nuget/v3/index.json.
NU1301: Unable to load the service index for source https://.../nuget/v3/index.json.